Where does this strange noise come from?
 

I am so exhausted with the noise. The car is 93' california version 300E with M104 engine.

Symptoms & information:

- The noise lasts no longer than 2 seconds.

- The noise is intermittent. I cannot make the noise intentionally.

- The noise comes only after minimum of 30-minute driving. So I think the noise is heat related.

- The noise comes only when the engine idles, the transmission engaged(D/R/3/2/1), and the brake applied.

- The noise comes from the driver side of the engine room. The noise is loud in the cabin, but I have not heard the noise outside of the car because I could not duplicate the noise.

- A kind of low-pitched sound(around 100 ~ 200Hz maybe).

- The most similar sound is the noise of a pneumatic/hydraulic pump.

- A/C does not affect the noise.

- Steering does not affect the noise.

- The idling of the engine is very smooth. No vibration.

Anybody help?

I have a similar sounding noise on my 560sel. It is the self leveling system hydraulics. I can hear the pump fluid pulsations inside the car. Not outside. Sounds like a groaning noise. Does your car have the SLS system?

Try upper bushing of the belt tensioner damper.

On my C230 the air pump does a self-diagnostic check once a day in the morning and it fits everything you describe although it occurs within the first 10 minutes. After driving about 5 to 10 minutes, when I come to a stop light it turns on for about 2 seconds. The engine stumbles briefly at this point and I can faintly hear a vaccum cleaner sound. It worried me at first until I learned what it was doing. Still seems kind of odd. I don't know if your car has that system though. The other ideas below look interesting also.

Mike

Update:

Thanks for the opinions. Unfortunately, I have not found the source of the noise.

dpetryk: My car doesn't have hydraulic pump...
Arthur: The damper is in good condition. No vibration.
mpolli: The noise starts after the first 30 minutes... And the sound is not as smooth as the air pump makes.

Now I can dulicate the noise with 30% success rate. These are how I can duplicate the noise.

- The engine must be on or above the working temperature. (>80C)
- The noise comes only when the engine idles, the transmission engaged(D/R/3/2/1), and the brake applied.
- Push the power window button (UP) when the windows are closed.

I letted my wife push the window button to find out the noise source from outside... failed. I can hear the noise outside, but it is VERY subtle.

Since the noise is far more noticeable in the cabin and it is from VERY driver side(almost the driver side door), I think the noise doesn't come from the engine itself or something on the engine. It's from behind the left side of the instrument cluster or under.

One more thing. When the noise is noisier than usual, I can feel vibration, too. The vibration comes through the steering wheel and driver side floor.

Any opinions?

Is the car moving when the noise occurs? Worn speedo cable perhaps?

How about the noise that the central locking system vacuum pump makes? I realize that the pump is in the trunk but it's an idea. This pump runs for only a few seconds when it operates. Wires associated with its operation would be in close proximity to the window circuit wiring. Maybe an intermittant short between the the two circuits in the door somewhere. Check the locks and see what they are doing when the noise is present. They may be unlocked and the pump is trying to unlock them again for some reason. Locate the pump & check it when the problem happens.

Thanks for your opinions.

gmercoleza: No. The noise occurs when standing still.

dpetryk: Very interesting and original idea, but the vacuum parts are too far from the noise source...

What if the noise is a kind of rattle then what would it be? I checked the charcoal canister and the plastic protective cover under the engine, but they are solid.

The drag link ball joints are not in good condition. I can move the drag link without big efforts. The other joints are fine.

Any helps? The noise makes me #$%^&*

how about something with the booster or brake pedal assembly?

If it was me I'd be looking for it with the kick panel and cluster out and be sitting in the seat 180 degress out... i.e. upside down.

Thanks Johnhef.

I tried that upside-down method and I realized the noise source is not the driver's foot area but the transmission area. When I drive the car the direct noise is blocked by my right leg but reflected by the door and the footrest.

Since the noise is low and heavy, I'm mainly suspecting the exhaust system.

More opinions?

Post # 3

Wedge a screwdriver into the top bushing while someone holds the brake pedal while the car is at idle , in gear
Infamous fuaut on 104.

Hello

Near the brake booster you will find a number of vacuum hoses, the one you need to focus on is green and yellow I believe. I would try disconnecting the green yellow hose from the vacuum source, block or cap the open vacuum line with a golf tee then drive the car normally. With this line disconnected your AC system will not function pneumatically and it should default to defrost. If you do not hear the noise after conducting this test I would suggest checking the "vacuum operated" AC servos for leaks. Check the elements with approximately 4-inHg (inches of Mercury). By using a small amount of vacuum you will avoid sealing a pin-hole leak which may exist on the rubber diaphragm.

Good luck

Matinee,

I had such a rattle/chirping that drove me absolutely nuts! Never could see anything wrong, until I spotted the fact that my shifter lever (underneath, in transmission tunnel) and the linkage to the transmission where metal on metal - the plastic/neoprene (?) bushing had disintigrated completely - and was gone. As such various combinations of factors made the engine/transmission vibrate just right and produce a mystery noise. Check if out, hope that's it because it's about a US 85 cent fix.

Thanks for the opinions.

Arthur: Though I did not suspect the tensioner damper, I didn't find any reason not to check it again, so I tried the famous O-ring fix. Result? Nothing. The noise is still there...

MrCjames: I'm not sure I understand your idea. If I hear the noise when the climate control is shutted down with the "0" button, then can I rule out the idea?

ScotzBenz: The shift lever is very solidly fitted. I tried to grab the shift lever when the noise came, but there was no unordinary vibration on the lever. I wish I can fix it with 85 cents like you did...

buckwheat: Yes, I will post the solution once I find the source.

One more information on the noise: The noise is a kind of FADE-IN (gradually louder) sound. That means there is a resonance I guess.

Not entirely, you will need to eliminate the siwtch-over valve block as a possible source for your noise. Since the noise is heard inside the car I believe all the items in the dash need to be eliminated