2004 e320 wagon. I noticed a knocking noise that can only be heard in low ambient noise surroundings. (pulling into garage). The noise is more pronounced when the vehicle is in gear. I used stethascope and am positive the noise is coming from the transmission bell housing area on drivers side of trans. I took it to trusted MB mechanic and Dealership. Both told me carbon build up?? What??? I have been in auto biz for 19 years and never heard this before. Please excuse me as this is my first benz. Is this a normal noise??? Tried GM top engine clean as recommended by mechanic and no difference. I just need some direction without pulling out the transmission.