lov2seadoo

Knock sound only can be heard at idle or at slow speeds. I think it either goes away when I accelerate or I cant hear it over the engine. I was thinking the cat converters or drive shaft only because after lifting the car to see those are the only parts in the area of the sound.

tower

If your idling, I assume you are not moving...Therefore the driveline (ie driveshafts) are not turning. So it won't be that.

My only experience with cats was when my old volvo's plugged up. In which case the engine/exhaust whistled on increase speeds, until it plateaued due to back pressure being to great to let the exhaust gases out of the motor. It is possible that the ceramic catalysts are broken up inside (common). You can test this easily enough with the motor off hand shaking the exhaust system. If it sounds like marbles/stones trapped in a can, its a pretty good bet that your cats are kaput.

Does the sound go away when the motor is at operating temperature? The reason is my S500 "raps" at startup, generally the noise disappears once the engine is at temperature. I have scoped out the engine with a mechanics stethoscope and it is at the head (ie valve area) that the noise is most prominent, therefore I am reasonably confident my noise is the lifters.

I'd also do a timing chain stretch test. The earlier 420's and 500's (W140's time frame) had a tendency to wear the timing chain guides, letting the chain get loose on its sprockets. Once loose enough, the chain could slip a tooth causing a lot of damage. How many miles on your rocket by the way.

lov2seadoo

thanks for the reply.. i made an extra point in my original post that the sound is NOT coming from the Engine. Im not sure about how tight the exhaust system is on other S430/S500's but my exhaust is not shakable to hear "marbles" in the cats if the ceramics are broken. I was surprised to see that comment because I thought I could shake it and after lifting it and trying.. I just stopped and looked in awe impressed on how "beefy" mercedes benz's are made. Regardless i still need the answer to the sound..

tower

I understand from your original statement that you believe the noise is not engine related, but your original statement stated that the noise was present "at idle or slow speeds". Does the noise occur only when the car is moving or is the noise present even when the car is parked or stationary, engine idling? It does make a difference.

If you are convinced of the source, I can only direct you to something similar with exhaust noises:

http://www.benzworld.org/forums/w220...hlight=exhaust

see link #8 in particular of the above or maybe:

http://www.benzworld.org/forums/w220...hlight=exhaust

Best I can do with the limited information provided.

Ron

lov2seadoo

The knocking sound is primarily loudest when stopped at idle. then as I accelerate it knocks some but either smooths out..or goes away at higher speeds. Honestly I just cant hear it anymore after I get on the gas and accelerate. I hear it coming from under the passenger "cabin" or basically from under the seating area but outside and under the car. I lift the hood and hear nothing. If I get on the ground and listen from under the front grill area.. I dont hear it, but as I get up and approach the front doors I hear an inconsistant knock... knockkncock, knock, KNOCK KNOCK,......if I give it gas in park.. it definately goes away, but as the engine slows in revs it comes back. Hope that helps figure it out.. please let me know what you think

tower

Alright a little better in clarification. Thank you.

As I said in my first post, if the car is not moving then the drive shafts are not turning. So expanding this so that we can definitely remove the drive line in the equation, with the engine on and the car in neutral or park, do you still hear this knocking noise? If the drive shafts are not turning I personally cannot see how they would be knocking.

I also realize that you have tried to "shake" the exhaust system and found it to be very stiff. I can respect this because owning (2) of these beasts I can attest the general integrity of the system. But bear with me. When I say shake, I mean getting under the car and violently attempting to move the exhaust system by hand. Up - down, laterally, horizontally really give it a shake and try to set up harmonics if you can. Caution, make sure the automobile is secure when you do this. On my W220, the exhaust sections behind the cat's are secured with rubber mountings. So just double check to make me happy.

Outside of this, without seeing the vehicle, its a cr*pshoot to try to determine the cause of your noise. The good news is the vehicle doesn't need to be moving so with a mechanics stethoscope, you might be able to determine where the noise originates by trial and error.

I know I'm treading on a dangerous line, but I still get the feeling that the noise is related to engine speed....true or false?

Good luck and keep us posted with your progress.

Ron

da boss

I had similiar problem and it was my cats!!! Or something can be loose..

DrRidaz

I fixed a customer's vehicle today, if you're experiencing knocking noise playing "nuck nuck nuck" consistenly (you can almost play the sound with your mouth if you say "nuck" 10x quickly) coming near the engine (the sound isn't coming FROM the engine), then it is your airmatic compressor pump. It should be around $350 for the OEM part, but you can get it $100 cheaper from Ebay.

96_C280

Did you ever find what the problem was? Just bought my car and noticed the same scenario tonight. Hope it's not that pump ;/

tusabes

Most likely it’s a pulley or belt tensioner . Take off the serpentine belt and see if it goes away . If it does you use a mechanics stethoscope to pinpoint which pulleys are noisy

96_C280

Ok, Thanks.