Hi. Thank you in advance!

Needing some help diagnosing a sound, in case any of you have encountered this on your cars.

II have a 2014 E350 4-Matric Station Wagon. It has 19" Mercedes multi-poke wheels, with a +35 offset, staggered set up (factory is +48). I bought it this way, and am only mentioning it to give a full picture of the car. (I just ordered all new original 17" Mercedes wheels and new tires). The car is very well maintained with 150k miles.

This past 2 months, the car started making a sort of "buzz" sound, which sounds like a rattly electrical pump almost. Almost like a tiny air compressor pump. I have a small cheap plug-in 12v air compressor and it sounds somewhat similar to that. But it's not necessarily electrical, it just sounds similar. It only happens when the car is in gear and rolling, not when stopped and idle. It's pretty loud, or at least very easily noticeable on the outside of the car. It is not the rear Air Suspension pump, we have ruled that out since it is demonstrably quiet and the rear suspension pump activates in park with the car idle, unlike this problematic buzz. I can hear the suspension airbag pump activate and its quiet.

This problematic buzz only happens when the car is driving, NEVER when in park, nor when stopped in gear and idling. When I am driving, the buzz will start randomly, at street speeds. Cannot hear it at highway speeds. I can feel a slight vibration under my feet, and as I roll the car to a stop, the sound goes away as soon as the car is stopped. When the sound starts, I have shifted through all the gears manually and nothing changes, the sound remains consistent. Even if I rev the engine while coasting, the sound remains consistent. I have also placed the car in neutral and let it coast and that also does not affect the pitch or frequency of the sound. Coming to a complete stop definitely eliminates the sound, and it doesn't re-trigger until you're driving for a bit. It never initiates when starting the car from a stop, only when its already driving. And sometimes, the sound starts right before I come to a complete stop, as I'm pressing the brake, but then the buzz goes away as soon as the car is stopped. The sound also stops on its own after a few seconds of driving, like after 15 seconds, it usually stops, though sometimes it goes longer.

Yesterday, I drove it until the sound started and I manually turned OFF the car and let it coast to a stop, the sound continued even with the engine completely off and only went away after coming to a compete stop. The buzz doesn't seem to change in pitch or frequency, as if something is rubbing along a rotating object. It seems to keep its frequency throughout, no matter how fast or slow I'm driving. That's why I characterized it as almost a compressor pump like sound. But again, it happens with the engine off, as long as it has initiated with the car driving.

Other clues, it seems to be coming from under the car, toward the front drivers side, but that's because I am seated there. I have opened the door to listen and that's where it seems to be coming from. Another peculiar thing, is although the pitch or frequency doesn't seem to change based on the speed of the car, I can get the sound to "hesitate" or pause for a split second if I accelerate in gear or decelerate. The sound hiccups a tiny bit and continues as I do this repeatedly. Still, only coming to a stop will definitely cause the buzzing to go away. Reminder the the buzzing sound can be described as if a pump is being actuated. Another clue, occasionally the sound has initiated when going over a bump, but definitely not every bump causes it. Usually, it just starts while I'm randomly driving. It happens every time I drive it, usually when at operating temperature, but it has definitely happened when driving it cold, without the car reaching full temp.

A Mercedes specialist mechanic had it for two days, could easily replicate the sound, but not on the rack, only when driving. He thought it was the ABS unit on the inside of the engine bay. He manually triggered that ABS pump, but it was quiet, as per specs. He is baffled. Placed it on a specialized mechanic forum and he got little to no help.

I have ordered new stock original Mercedes 17" wheels in case the oversized wheels with the wrong offset is causing some system to trigger. Or maybe it has caused some bearing damage? Who puts a staggered set up in a 4-matic anyway?? jeez. Can bearing damage sound like a small compressor pump?

Anyway, praying to the forum gods that one of you might have heard of this.

Thank you so much!

 

Eagon have rear Airmatic right?
A compressor is involved... just what you're after, right?
Can you investigate with your MB scanner??

 

Thanks for your response. Yes the Wagons have rear airmatic. But as I mentioned in my post, the air compressor is not the problem. We can manually trigger that compressor and it is very quiet. The airbags are relatively new, so they do not deflate enough for that compressor to run repeatedly. And also, this sound ONLY happens when driving, not when stopped. The airmatic compressor runs whether your driving or idling. It's not the aromatic compressor.

Thanks.

 

have you surveyed your chassis lately?

May be that would help you save time with insight on module status. These chassis are computers on wheel !

 

I have taken the car in to get scanned. They found no errors whatsoever in any component. Engine and transmission etc have no errors.

 

Your car being fault free is really good news for it run well. Did the specialist have an educated guess about your issue?

 

SI, it is a 4-Magic with staggered setup?!

How bad is the tire diameter difference front to back?

You may be hearing the center differential lock disk grinding as it wore out with speed difference between axles…

If this is the case the noise goes away if you install same size tires on both axles.

 

thanks for the reply.

I suspected the staggered wheels had something to do with it, so yesterday I installed the original 17”s with all new tires. I bought the wheels specifically for this issue last week. A fully stock square set up. The noise still happened in the same exact way I described in the first post. So bizarre.

perhaps the lock disk you mentioned got damaged and is still complaining? Can you describe the function of this disk you mentioned? Is there a 4-matic process that happens when you’re driving normally that perhaps is damaged when it activates?

the mechanic had no idea nor made an informed guess. He said he thought perhaps the ABS system was at fault, but when he manually triggered it, it was pretty quiet.

 

Mercedes has changed 4-Matic many times between cars and I’m not sure your 2014 model has this but the one I’m talking about is a spring loaded disk that provides locking between axles. You need the lock as without it your car would not go in slippery conditions when wheels on one axle lose grip.

This spring loaded disk is the reason why these 4-Matic cars come with square setup so there is no speed difference between the axles that cause the lock disk wear when wheels on both axles have good grip on road, which is most of the time.

Some newer cars Mercedes builds as FWD that have clutch pack that connects rear axle when slippery conditions are detected but I think your car still use the spring loaded disk that is always connected.

With square setup there is no movement in the lock disk so if you still hear it then it is not the lock disk. You may have problem with the “other” typical 4-Magic part, which is the planetary gears of the transfer case if the noise comes from that area of the car.

Any chance to record the sound for us?

 

Another thing that could do the noise is the propeller shaft touching the aluminum shield under the shaft. It is though that the U-joint knuckles are the biggest diameter and so would make sound that changes frequency with the car speed.

I had this happen with my S550 after MB replaced the flex disks and they had to remove shield for the job. Re-install the put shield to contact with shaft. Fix for it was to pull down on the shield.

This sound comes and goes because the center bearing of the shaft is mounted in a rubber holder that allows the shaft move radially and over time the rubber gives and shaft lowers at the bearing coming closer to the shield below it.

Could there be a round part of the shaft outside the joint knuckles touching the shield so there would be no frequency with the sound?

 

I’ll lift up the car today to see if I see any shield touching the drive shaft.

i did record the sound. Here it is attached. I opened the door and came to stop. You can hear it wind down as I stop.

 

The sounds also changes a bit if I accelerate or let off the gas, as if responding to the shift in load caused by the increase and decrease of engine torque

also, sometimes it is directly caused by hitting a bump, though not all bumps and sometimes it starts without hitting a bump.

 

Have you gone around and inspected all your wheel arch panels, underbody panels, driveshaft covers, heat shields, etc? Look at everything twice and try to pull on it and move it around, see if it's loose even if it looks right.

That sounds almost like the sound we would make as kids by sticking a card in our spokes. Like a corner of something is sticking into a rotating part. Maybe...

 

as soon as i got home, I lifted the car and checked for loose parts, and I found this loose screw. Pic below. I am going to tighten it and drive around.

when I tap on that plastic shield, it does make a similar buzzing sound cause the washer oscillates against the screw head.

 

After tightening the screw and also adding a missing screw to the other side, same location, I drove it around and the sound is still there.

I’ll poke around some more.

one additional observation, I can make the sound happens if I accelerate past 40mph, and then let off the gas allowing the wagon to coast in gear. A few seconds later, the sound starts. If I accelerate a bit, that sound hiccups. After this tiny pause or hiccup, the sound will return. But if I pump the gas a few times as if to accelerate fast, the sound goes away. Almost like I jostled something back into place.

 

Tightened all loose shields. Still makes the sound.

I couldn’t see the drive shaft cause it’s covered by shields.

maybe at this point I have to take it to the dealer and pay for a diagnostic.

 

Man, the MB expert already told he is lost with this…

So, as you are hands-on person , take the aluminum shields down from under the drive shaft and go drive it around.

From the last couple posts it sounds more and more like the propeller shaft is hitting the shield.

I have not been able to hear your sound post yet though.

 

ok. I can do that tomorrow. Can you tell me where the propeller shaft is? I see a black flat pan just around where the transmission Is.

If you DM me your email I can email the recording to you.

 

The propeller shaft is the driveshaft.

 

Like ChuangTzu said the propeller shaft is the drive shaft. Mercedes calls it that.

I was able to hear your recording and the sound DOES HAVE frequency that changes with the car speed. It is very high frequency though and I'm not sure the U-joint knuckles would give this high frequency but as a troubleshooting step I would remove the shield (two pieces in my S550) under the shaft and see if noise goes away.

 

You’re right, at lower speed it does change pitch as you come to a stop. But at normal speeds say above 20 miles an hour even at freeway speeds. The sound seems to be the same frequency just FYI, as I go faster on the freeway or on the road or slower it doesn’t seem to change much until I come to the stop

 

Update.

after tinkering under the car pulling on shields and tightening screws, the sound didn’t go away.

one morning the sound was there but more faint. So I decided to run it at 100mph in a safe empty road for about 1 minute.

then the sound has not come back after a week.

i wonder if there was something stuck on a rotating part that got unstuck or flew off at high speeds.

thanks for helping. I have no idea what it was but it’s gone now. Crossing fingers it doesn’t return.

 

Good news it is gone, but if it is the shaft hitting the shield the noise likely returns when the rubber center support bearing holder sags a bit more and shaft lowers.

If the noise returns take shield completely out and see if that makes noise go away. If it does, then you know FOR SURE it is the shaft hitting the shield. Fix is to replace center support bearing and holder.

 

thanks! I took a screenshot of your reply and will text it to the mechanic. He is also baffled. But this makes sense.

will keep tabs and post any update.

thanks again!

 

Ahh the good ol ziptie around the driveshaft prank