Hey, all, I'm new to the forum, so forgive me if someone has mentioned this before, but I'm not coming up with any hits in the search.

I just bought an '01 C320 Sport from a private party and I'm having a problem which is baffling me. During deceleration ONLY between 60-65mph, I'm getting a hum that sounds like it's coming from the differential. It kind of sounds like when you manually downshift into the wrong gear, but I'm not seeing increased RPM. (Obviously it's an auto.)

Any idea what this could be? The dealer replaced the fuel filter and upgraded some harness and said the problem was fixed; two minutes later on the freeway, it was still doing it. The odd thing is that it ONLY happens at about 63mph, any faster or slower it disappears, and it doesn't happen while accelerating in that range. I've been trying to get an appointment with the shop foreman to ride along with me, but every time I go in, they tell me he isn't available, etc. (The dealer is less than helpful, but that's another story...)

Any suggestions you could make would be greatly appreciated! Even if it's just a comment that this is normal. I've owned several BMWs, but this is my first Mercedes, so I'm still getting used to its little quirks.

 

... that the mileage is only 12,000 so it can't be a typical high-mileage differential problem.

Interestingly enough, my loaner C240 had the same problem during acceleration at certain speeds and it too was only about 12,000 miles.

 

I read an MB tech bulletin that described your symptom perfectly. The differential is actually hitting the frame. The fix is to shim the differential mounting points.

 

mleskovar, you're a lifesaver, thank you! Now let's see if my idiot dealer (er, the brilliant individuals who service my fine automobile) can figure it out without me having to spell it out for them. Any idea where I can find that tech bulletin?

 

Can't find it in my stuff....try searching old posts. If I remember correctly I saw it about 1 1/2 years ago. There are some lists posted occassionaly with all the tech bulletins but they don't always list the TB#. Good luck.

 

Okay, I'm thrilled now. Four days and a new differential later, no hum!!! Of course, it took going to another dealer to resolve it, and needless to say, I am sticking with them for service from now on. No problems getting them to change it out like others have mentioned, but all the input from the forum definitely helped pinpoint it, thanks! My new service advisor thinks I'm the rare female customer who knows cars, lol, I'm not telling.

A little off topic, turned out I had 3 collapsed lifters as well, which I had been complaining to the other dealer about, and they told me they couldn't duplicate the problem! Between the diff and the lifters, it feels like a different car!