rdoty

My 2005 S430 has recently developed what I can only describe as a "putternig" noise during acceleration load. The engine idles normally and runs normally but when I accelerate there is an increasingly loud noise with a frequency maybe equal to detonation frequency if that makes sense i.e. a "putt" each time a cylinder fires. The check engine light is not on. I surmised that it might be a bad EGR valve but that's not one of the characteristics. I recently drove a 1000 mi. round trip with no other problems although my mileage is down about 3 mph. Any thoughts?

redaotrosh

I notice that in my car but only during reverse.

wallyp

The first thing that I would look for is an exhaust leak.
Look for any visual stains showing leakage.
Get six feet of 1/4" rubber or plastic tubing. Put one end of the tubing to your best ear. With the engine idling, use the other end of the tubing as a probe around each exhaust manifold gasket to listen for the puff of a leak.

rdoty

Thanks for the suggestion Wally. That does make sense and I'll give it a try this week.

rdoty

Definitely not an exhaust leak. Is it possible the noise might be coming from the 4Matic? The car drives normally and there are no vibrations. I can't hear anything unusual with the car idling or the hood open, only when the car is accelerating or under load i.e. constant speed from maybe 30mph up.

wallyp

The only way to diagnose the problem is for someone to listen to the noise and determine the source. It is very unlikely that an "internet guess" will be useful. That being said, there are a lot of possibilities, including, but not limited to:
- Failing flexible disk on the drive shafts.
- Exhaust pipe or muffler touching the body.
- Failed motor mounts.
- Exhaust leak - it is really difficult to positively prove that there isn't a leak.
- Failing CV joint.
- Leaking spark plug (possible, not not common).

Try to determine if the noise is at engine speed (changes with transmission shifts), or drive-line speed (doesn't change with transmission shifts).