E320Star

Would a bad catalytic converter cause a tapping sound under the hood under the engine on the driver side?

lexrex

Bad cat sounds more like a bunch of rocks rattling inside a can.

Does the tapping sound more like a clicking noise? If so, it could be your purge valve up near the ABS/brake module by the shock mount... The purge valve noise is normal.

MARK CUMMINS

Could be a lifter and or a defective camshaft...If from the engine..

purplegrog

also don't forget that your emissions warranty should cover a bad cat (or in my case, both cats going bad) up to 80,000 miles. You definitely want to go this route if you can; I was quoted roughly 900 dollars each for left and right catalytic converters, just for the part.

E320Star

I think it warrants a dealer visit now. Thanks all

mike430sport

I had that problem. Yes it is the Cat. First mine started with the passanger side, changed it. Then the driver side went out. When driving it sounds like an exhaust leak, tick tick tick tick. Also you hear the metal rattling. The Cat breaks up inside and the pieces start moving around like a popcorn popper . If you put your car up on a rack you can hit the Cat and hear that it is busted up inside. It is very expensive to change, I did the research. Only if you buy online and tell the people the car is Canadian, the Cat will be cheaper, diffrent emissions there, same pipe. I got lucky, found a 2001 E55 AMG that was parting out and the guy sold me everything. The AMG pipes are bigger that the stock. I got both Cats, resonators, and muffler for $500

C-2

I had a rattling sound, and also thought it was a cat. But my sound only happened while the car was in drive - not park. This led me to look further and after several hours of poking and prodding - it turned out to be a broken weld on the exhaust pipes. Specifically a small cross member separating the pipes. Couldn't find it just looking; rather, had to move the pipes around before I saw the break. My neighbor welded it up and saved major $$$$ since I'm sure the stealership would have replaced the entire cat.

I have pics if it's still an issue - but see if it's an in/out of gear problem first.

E320Star

I finally took it to the dealer. The knocking sound was coming from the driver side behind the front wheel. The MB dealer checked both Cat coverters on both side. He discovered the passenger side Cat converter is the colbret. Even thu I was sure it was the driver side being the bad one !! So, they replaced the CAT converter on the passenger side free of charge since it is still under warrantee 8/80,000. That was last Friday 3/24/2006. However, the sound did not go away and it is still happening from the Driver side. It only happens when the car is cold. He told me the way to know if it is the cat, is when you start it up in the morning while the car is cold, when the idle drops after 1 minute after start it, then you will hear this knock/tap-ing sound. When he checked the driver side, he said it was not noisy when he tap-ed it while checking it out, the passenger side did. What should I do, could it be the driver side cat still? Thanks