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?

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

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

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

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?

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

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.

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

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 #$%^&*

I fixed it finally. It was an ignition coil. The coil went very bad recently, so I changed the coil then the problem was gone.

The coil was not completely gone but it varied its secondary resistance along with the temperature. The coil was original with a blue sticker on it.

Thanks everybody helped me.