Polo35m

Hey guys I just picked up this car a few months ago and upon startup whether it's hot or cold doesn't matter there is a slight clunk/thud sound coming from the rear of the vehicle.
I did a little research and some people have a similar issue which was apparently the exhaust flaps opening and closing. Some describe it as a barn door opening and closing mine unfortunately is not this issue. I actually tried disconnecting the flap vacuum lines and also wiring them shut and wiring them open so by process of elimination it's definitely NOT the exhaust flaps. But it is definitely a subtle clunk when the car is started. Any ideas would be greatly appreciated!
By the way this again happens when the car is hot or cold and of course in park.

BOTUS

a bit more info on when would help - are we just on start up, or driving off ?

I'd check if the silencers are free to move and are not hitting bodywork as the engine moves under cranking / as it fires up (that said, are the engine and transmission mounts doing the right thing),
if that seems good have internal welds broken and you have baffles rattling about?

Polo35m

Yes sir thank you so much for the reply! So it is on Startup wether cold or hot and sitting in park. And it is literally just a single Clunk/thud And that's the end of it It does not persist. Then you shift in gear and drive off normally everything is perfect no sounds. I did get the car on ramps and check for exhaust brackets Donuts Etc or cracked welds and I can't find anything. It's definitely in the "rear" of the car. One of my theories is possibly the inner muffler material is disintegrated and thumping around on startup? It doesn't do it every time but it does it most of the time. I was so sure it was related to the exhaust flaps but by isolating them and disconnecting them I have eliminated that possibility Although another theory is it could be the device which delivers vacuum to the exhaust flaps it is sort of hidden there are only two vacuum lines coming out of the rear bumper area but I have not been able to locate or trace where they originate. That is my next step.

BOTUS

worth a check - open the boot and get someone to listen as you unlock, again when you put the key in, and again when you turn to fully on

there's a lot of pointless junk popping whirring and doing its thing in there (mostly left rear hinge area)

doni01

iTrader: (1)

Mine does it as well. Exactly like yours. Same clunk/thud on start up.I just can't pinpoint it where it is coming from. My front suspension is new including my motor mounts so highly doubt it is coming from the front. I also have the S63 exhaust and my valves are always open so definitely not from there. Like Botus mentioned, it will be hard to find...

Polo35m

Man it is absolutely driving me crazy! My latest theory is that maybe it is some sort of actuator when the engine is started that controls the vac to the exhaust flaps but I just can't find it or get at it if it even exists like I said earlier I dropped the bottom round metal cover under the spare tire and there's literally two vacuum hoses coming out of the body! I don't know where they go but I'm going to keep delving in

doni01

iTrader: (1)

Yeah it drove me crazy as well but I have just been ignoring it. lt happens about 90% of the time and my car stays in the garage where I can hear it but don't have anybody that can just be there while I start it. I highly doubt it has anything to do with the exhaust valves though...

Polo35m

Well your response has partially inspired me to keep trying to figure this out. I was so sure that it was the exhaust valves because I found a similar thread on a different forum where he described it as sounding like a barn door closing and opening but it was not the case with mine. I even disconnected the vacuum lines going to the exhaust flap actuators and tried plugging them and tried leaving them open and still the same sound. I'm going to start listening at the back and have my wife start the car randomly to see if I can at least isolate closer to where the sound is coming from but it definitely is the rear of the car

doni01

iTrader: (1)

I was actually going to open up a thread while back here as well since I did a lot of research with no success. But the issue is, there could be so many things that are in motion when starting the car and figured if it was an issue sooner or later it will get worse and then it will be easier to diagnose. But it is exactly how you described it. Clunk/Thud!

Baltistyle

Do you have abc on this car? It could be the sound of the valve block solenoids opening when the car starts. The valves close when the car is off so it won’t lose fluid if there’s an issue outside the block.

See if the noise is close to the rear valve block. See if you can replicate it with the raising button.

doni01

iTrader: (1)

Nope, it is Airmatic...

Polo35m

Same here it's just the basic airmail suspension no ABC mine is the S550 base model 2012. I plan this weekend to tear the trunk apart and try to locate where these two vacuum lines originate from my gut tells me that somehow they're related. I don't believe it is air suspension related either in the morning I walk around the car and make sure that it hasn't sagged and then when I start the car I take note to make sure that it's not raising and again it does it whether the car is hot or cold

doni01

iTrader: (1)

I truly believe it has to do with the car shaking on start up...it could be anything. Mine is a 2013 4MATIC with only 65K ...

Polo35m

I'm at 70k I guess it could be the rear transmission mount but man it sure sounds like it's at the absolute rear of the car maybe I'll get it on ramps this weekend and check it out. We both have quite low miles for the year but from a Time standpoint 11 and 12 years it's a long time. We'll report back ASAP

Polo35m

You can drive yourself batty thinking of what it could be but I did just think of a possibility maybe it is the fuel pump in the tank because it does kind of sound like it's coming from that area. Maybe I'll yank my rear seat this weekend and start the car and listen

Polo35m

Well I'll be damned! So this morning rather than just starting the car I got in and pushed the start button twice and it made the sound! So this is telling me that it is not engine starting related as in like engine mount or exhaust or anything like that and I believe that it was coming from under the rear seat so I'm thinking fuel pump. This weekend I'm going to pull the seat and replicate it again and try to zero in on where the noise is! Keep you posted! Been quite a while since I replaced a fuel pump in a Mercedes but I know they're on top of the tank under the rear seat and there are is a lot of Plumbing going to and from it so maybe it's just one of the fuel pipes knocking?

doni01

iTrader: (1)

That is why I kept saying that it is not the exhaust valves as well. It is more in the middle of the car then at the end of the car. My motor mounts and transmission mount are only a year old and were replaced with oem parts so definitely not those.
Today I had my daughter trying to listen as O started the car. First it did it just like yours with only the push start without starting the car so don't know what wakes up when the push button is pressed without starting the vehicle. My daughter pointed that the noise came from behind the engine...Now you got me investigating again lol I will report if anything new as well 👍

Polo35m

Interesting so you believe yours is coming from the engine bay. Mine is without question coming from the rear of the car but I still cannot pinpoint it. It doesn't appear to be the fuel pump I removed the back seat and listened with my stethoscope I then got excited thinking that maybe it was the purge valve/charcoal canister over the right rear fender well
I checked that this morning while my wife started the car and now it seems like it's coming more from the center rear of the car. Is just unbelievable. Next step is I'm going to remove the panel that covers the consumer battery and all of those Electrics in the trunk and lay inside of there tomorrow morning while my wife starts the car. I will report back.

doni01

iTrader: (1)

Yes mine sometimes comes from the back of the engine and then sometimes from the middle of the car... That is why I gave up and figured that I will be looking at it when it is more pronounced and easier to spot ....

Polo35m

Well I figured I would report back with my findings. So again mine is definitely coming from the rear of the car but I still haven't been able to pinpoint it it's so difficult when you're listening but it is absolutely in the rear of the car and I believe in one of the fender Wells. So I just started disconnecting things and testing to do process of elimination the first thing I did is I disconnected the trunk latch because actually it kind of sounds like that unfortunately that was not it. The next thing I did was disconnect and bleed all of the pressure from the air pump in the trunk which controls the dynamic seats unfortunately that was not the problem. The next thing I decided to check was the exhaust flap vacuum lines and actuator mine have never worked since I bought the car so I was pretty stoked that that might be the problem. What I found was the main vacuum line from the actuator to the Y which then goes to each exhaust valve was completely disintegrated holding no vacuum at all. To access it all you have to do is remove the hard plastic trim that the trunk closes down onto and you don't have to completely remove but you pull back the right rear passenger side inner trunk liner and you will see the actuator mounted with a rubber mount. The vacuum line was completely disintegrated again from the T all the way to the actuator. Then I unplugged the actuator from the wire harness to check to see if that was the sound and unfortunately it was not. I replaced the vac line and now my flaps work. So now I'm back to square one. The next thing I'm going to test is the purge valve on top of the charcoal canister for the fuel vapor canister. That will be this weekend's project. Definitely keep you posted.

Polo35m

Well I couldn't wait for the weekend I went ahead and removed the fender liner and dismantled the charcoal canister and it's solenoid and electrical plug and to my astonishment the sound is still there on Startup. I just can't pinpoint it's location you would think that as many times as I've heard it I'd be able to zero in on the area but I can't tell if it's middle left or right rear still going to keep at it however. We'll definitely report back.

BOTUS

do you use the park brake ? and do you park on a slope - this can cause it to creak/groan if it moves slightly

mine has this magic I feel like releasing at times - I can apply, feel it bite, feel it hold the car, then as you slip in to P you feel the park brake release a smidge... one in 10 times it seems to do this and at other times it doesn't hold so you press the switch again and it holds firmly - I'm wondering if it fills with dust and this gives strange behaviour - tried to get water there with a hose but doesn't seem to help - done the set up and check behavior with diagnostics makes NO difference - something tells it to release

Polo35m

Absolutely appreciate the possibility and the advice unfortunately I live in Florida where it is absolutely flat and I never ever use the parking brake. But I have a couple of new leads a friend thinks it could be the fuel pump and another friend believes it's something internal of one of the struts so that is the project this weekend armed with a stethoscope and my wife starting the car periodically I'm going to investigate those. Keep you posted

doni01

iTrader: (1)

I don't drive the car every day and most of the time it is on a Ctek BatteryTender. One thing I noticed is, when I first plug the Tender, I do hear the little bang coming from the passenger rear seat. So definitely that makes me think it might be the fuel pump back there but the clunk on start up is more of a metal sound...it could definitely be a shock or even a starter maybe? I know you say it is in the rear but the noise travels so that might be some to look at as well...

johnnyrocket52

A suggestion for pinpointing the sound: Take the car outside in the middle of a large parking lot, 10 meters away from anything (poor man’s anechoic chamber) where the sound cannot echo off of anything.

Open every door, hood (bonnet), and trunk (boot). Station 6 sober car type buddies (with decent hearing) up close all around the car, and test.

JR