Hi guys, this makes my inaugural first post since I am typically a lurker, and have found what I have been looking for using the search function.

Sorry for the long-winded post, but I hope you guys have some ideas on what this could be, because I am wits end here.

Car Details:
2005 C55 AMG
75k on the clock

I've exhausted my search here on the forum, and others, and I am still at a loss to diagnose this unbelievably annoying buzzing sound inside the cabin when my engine revs between 4000-5000 RPM.

It sounds like it is coming from the drivers side dash -- although I just can't figure out exactly where it originates from. The bane of my existence sounds like two thin pieces of metal vibrating together at such a frequency that you can say that it sounds like an amplified bumblebee. As far as I can tell, the sound is mechanical (two surfaces rubbing together), not an electrical whine or interference. The radio and speakers have never been touched, and this noise has only surfaced during the last 6 months.

This sound is only ever present when the engine is rotating between 4000-5000 RPM. Not under, not over. The sound also comes and goes, as does my patience listening to it. Let's face it -- I didn't get this car to drive it like it's part of a funeral procession, so needless to say, I hear this sound more often than I want to admit.

My patience is running thin, and my mechanic says to diagnose this sound will entail taking apart the entire dash (3 days work) and will cost me some big bucks. Quite frankly, I have given him enough money already, and want to ensure that I have exhausted every possible avenue prior to having "open heart surgery" on the dash.

I am almost at the point where I am going to break down and give the stealership my money just to rid myself of this ultra-annoying problem, so this is my last ditch attempt at bringing closure to this issue.

Earlier I actually made a video with my iPhone, but with the incredibly hi-fidelity mic on the iPhone it doesn't pickup up this buzzing effectively with the roar of the engine.

Looking forward to any bones you can throw a guy over here to preserve my sanity.

Cheers

 

Are you pretty sure it is coming from inside the cabin (behind the dash) and not from the engine (sound making its way into the cabin). Does it make the sound if you rev the engine past 4,000 rpm's while stationary? If so, can you have someone do that while you have your hood open and head in reasonable proximity to the engine compartment?

Just trying to offer some suggestions to narrow it down. It's certainly unusual.

I would worry about pulling apart the dash as I don't know how they would recreate the sound with the dash out of the car (i.e., would they really drive the car and get it up to speed with all the interior stuff out of the car)?

 

Hey, thanks for the reply.

The sound is defintely from inside the cabin, and is only heard when the car itself is in drive, so revving in neutral while the car is moving, or revving when in park does not produce the buzzing.

When I am in first or second gear while driving, and the engine is revving between 4-5k this buzzing can be reliably reproduced.

I had the motor mounts replaced a couple months ago, as there was excessive vibration, but the buzzing sound persists.

It almosts sounds like two metal tubes loosely joined together, and coupled with excessive vibration, a la engine revving at high rpms, triggers them to vibrate.

 

Engine mounts were my first thought, so how about the trans mount? mine is shot at 71k...if you have your hand on the shifter when you hit a hard bump, youll feel the shifter jump. only in drive??? how about the joints in the driveshaft? they are not a normal u joint, more like a rubber isolator...dont remember exactly what MB calls them though...

 

I've never seen anyone complain about anything like that in the time I've been here. However, I will say if you do end up paying to get this diagnosed/fixed, you're better off going to the dealer. They will have access to specialized listening devices and access to the tech documents that an indy shop probably won't. I don't think your indy really wants to look at things like that anyway since they told you they'd have to start by pulling the dash,so they were probably trying to scare you off on that repair.

 

Thanks for your feedback. You make a good point.

 

Thanks a lot mkonei, these are good things to check as well. Appreciate it.

The shifter doesn't move when going over bumps, and I just checked some receipts and I had the transmission mounts changed with the engine mounts apparently.

The driveshaft joints -- will have to get this looked at as a possible suspect.

 

Not sure if it applies to the c55 but other w203s are known to have a rattling/loose noise behind the left dash/instrument cluster area. It is caused by the engine harness that is routed under the hood, there is a sleeve which holds it in place that is actually vibrating which emits into the vehicle. There is a DTB about this

 

Hey there Ricecooker, thanks for the info. I think we are getting closer to the source of this issue.

What you are saying makes sense, and I am guessing this is probably the cause.

Forgive my ignorance, but you mention "DTB" -- which I assume is a post somewhere concerning the aforementioned issue here correct?

 

If the noise is RPM related, then it is not the drive shaft (and the rubber part is called a guibo or a flex disc).

This does sound like a loose heat shield. There are several heat shields in the engine compartment and under the car that could make this sound, but given that you are sure the noise is from inside the cabin, I think Ricecooker has your best bet. But don't discount your shop's initial response. My first thought was "he's going to have to remove the dash".

 

It's surprising how the engine can make sounds that "seem" to be coming from inside the car. However, Since this sound is rpm based I have to assume the engine itself is responsible. the engine is vibrating, and the noise is being transferred into the car for some unknown reason. Could be heat shields as someone pointed out, or the wiring harness sure. But you have some pretty expensive motor mounts there that are designed to absorb that engine vibration so that it can't transfer noise into the car. I've changed mine twice now. the first time I used cheap eBay motor mounts and they seriously didn't last 2 weeks. They are DIY if you are patient and have jacks and tools at home.

My recommendation is to put a jack under your engine and lift it up and see how high it goes in the engine bay before the car starts moving. If the motor goes up more than an inch or so then the mounts are not supporting it anymore.

There also may be a misfire at that rpm that's making the vibration worse. Perhaps new spark plugs are needed as well?

best of luck!

 

Thanks very much for the great advice guys. I just had genuine MB motor and tranny mounts put on recently, so I would be shocked if I blew those out in such a short time. Heat shields are definitely something to look at.

I'll get my mechanic to toss it up on the hoist and see if the motor mounts are effective or not, since as you mention there Mike, they are designed to absorb vibration, and they may not be cutting it.

I really appreciate the responses thus far, and once I figure out a fix I'll share this with everyone.

Thanks.

 

Ive seen cases where the hard fuel line or brake vacuum line rub somewhere
and create noises inside the cabin. I have also see a stressed exhaust system create some odd noises/vibrations. Just loosen the bolts holding the cats and the mounting bracket and tighten everything up again. IIRC the DTB listed for the wiring harness only affected 272 engine cars but I can double check tomorrow at work.

 

Flex disk.