I am writing this post because this specific noise symptom was frustrating to find, resulted in my mechanic believing I was crazy, and caused me to perform maintenance I may not having performed otherwise. Hopefully this information can save others from time consuming searching and frustration. This occurred on my 2003 C240 4Matic Wagon.

The symptom is a "thump thump thump" or "knock knock knock" noise/sound that increases/decreases with the speed of the vehicle. The sound comes from under the center console/armrest area. Consistency varies. Sometimes it does not happen or is barely audible, while at other times it is very loud. Opening the armrest compartment intensifies the volume.

Sound scary? It certainly scared me. Fearing transmission problems I serviced and flushed my 722.6 transmission. When this did not fix the symptom I crawled under the vehicle for another examination and this time got lucky. It was definitely hard to see.

These automobile bodies are wrapped with sound abatement mat around the engine compartment, transmission, and undercarriage to cut down on road noise. On my vehicle (2003 C240 4Matic) the sound mat is warping (with changing temperature and moisture conditions making it difficult to diagnose) and contacting the 4matic front drive axle topside where it cannot be readily seen. The axle is perfectly machined except for one spot, a 5mm x 10mm raised square about middle shaft which is (of course) where the mat has chosen to collapse and touch the spinning axle causing the thump/knock sound.

If you are experiencing this noise symptom on your 4matic, I recommend examining the sound abatement mat along the front drive axle shaft for visible contact points. I fixed mine (maybe temporarily) using a small amount of auto trim adhesive between the mat and body. If the noise does return, at least I'll know its not the transmission about to explode.

Happy Hunting!