From new my C300 4m Sport had this issue.
On deceleration at 25kmh, 75 and 120 a loud whistling noise fades in and then out as the speed decreases. If you are on a downhill at 75 kmh and take your foot off the gas it will drone all the way down the hill. It does it in any gear any mode it is related to the speed of the car.

The dealer said it was most likely the transfer case which is integrated in the transmission so they replaced the transmission last week. I got the car back on Friday and it is exactly the same. I should have clued in when they said on the phone any noise you hear now is normal as we tested 4 other cars. Why did they have to test 4 other cars, obviously because there was still a noise.

This is not a minor issue this is quite loud and it is driving me nuts. I don't get why this should happen in a new 50k car, this would not be acceptable in a cheap car and even the B class I had for a week did nothing like this.

Monday will be judgement day I suppose. I wonder how they will deal with this now that it is "normal" in their opinion.
I cannot accept keeping this car as it is, they will need to repair it somehow or refund my money. I feel totally sick over this and I now wish I had gone BMW or Infinity
Anyone else having this issue ?

Benzagain, did you ask them if the 4 other cars that they tested were all 4Matics? I certainly hope so because we still have to hear the same issue happening to the non-4Matics W204s.

Hope the dealer will be able to resolve your issue quickly. Good luck.

Is this just a downhill issue? Because my car makes higher rev noises as I go downhill because the tranny won't kick down to a lower/higher gear... it almost feels like it got stuck in gear, but apparently it is supposed to do this (there is a sensor for it as I'm told) which helps when going downhill, preventing the car from picking up excess speed and also helps keep the brakes from wearing so quick.

I still don't like it - but my '06 4matic does this. Meh

- Eric

I bet it is the differential

I have noticed my car hold on to gears when on the round ramps or twisties. To me it just feels very smart.. There must be some kind of a gyrometer or something.

There are gyroscopic sensors that help determine engine braking, downshifting and such. The wine you hear is normal as the transmission is simply invoking something called grade logic. It simpy prevents the car from accelerating any more than it already is. Think of it as Hill Descent control found in many SUV's.

I have exactly the same problem on my c300 4matic, it has been sitting on MB's service dept for investigation for 3 weeks in my 5 weeks of ownership.

Their first attempt, which was instructed by German MB, they changed some parts (i forgot what those parts are) inside the transmission, result was, they failed, the whisting sound still remains on decelerating on 25 or 75km/hour.

At the time I was told about the result, I was totally pissed and had made up my mind to ask for refund and say goodbye to MB. I had a long talk with their GM and Service Manager and make sure they know how frustrated and dissatisfied with my very first MB product and ask for refund/replacement. Of course their job is to make their customers happy without taking their defected product back. I gave them another chance and more time to work on it.

Days later, they said German MB had instructed them to replace the entire transmission, as now, I am still waiting for the result....

After seeing another MB owner suffering by their defect product, I am kinda hoping this transmission replacement wont fix the problem. I already lost interests and confidence on their product, and will escalate to HQ for refund very soon.

<<From new my C300 4m Sport had this issue.
On deceleration at 25kmh, 75 and 120 a loud whistling noise fades in and then out as the speed decreases. If you are on a downhill at 75 kmh and take your foot off the gas it will drone all the way down the hill. It does it in any gear any mode it is related to the speed of the car. The dealer said it was most likely the transfer case which is integrated in the transmission so they replaced the transmission last week. I got the car back on Friday and it is exactly the same. I should have clued in when they said on the phone any noise you hear now is normal as we tested 4 other cars. Why did they have to test 4 other cars, obviously because there was still a noise. This is not a minor issue this is quite loud and it is driving me nuts. I don't get why this should happen in a new 50k car, this would not be acceptable in a cheap car and even the B class I had for a week did nothing like this. Monday will be judgement day I suppose. I wonder how they will deal with this now that it is "normal" in their opinion. I cannot accept keeping this car as it is, they will need to repair it somehow or refund my money.>>

Whatever this is, it probably isn't the transmission at all, despite what you have been told.

When a dealer replaces a component under warranty, the offending component goes back to MBUSA to a department where the thing will be tested to see if it really has failed. And, if MBUSA says it doesn't have a fault, the dealer has to eat the whole thing, parts, labor, everything.

What you need is an audience with the local MBUSA rep. Mentioning giving it back under your state's lemon law usually gets their attention.

frozenfish I sent you a PM.

It surely would be helpful for ALL posters to include at least their country of origin, if not more specifics on location.

Warranty and legal considerations vary dramatically from state to state in the US, and even more dramatically in other nations.

Asking for advice or posting a complaint without this basic information is almost a useless exercise.

I don't think OP or others were seeking legal advice, he was looking to see if others have the same issue.

and Warranty shouldn't matter from state to state..... but it would be nice to know the fab plant and production date (maybe a bad batch?)

At last, after long 3 weeks of waiting, I got the phone call from MB and they said they had replaced the transmission and the car is ready. But they did admit that the whistling noise at deceleration is not totally removed but significantly less. I went picked it up, drive around a bit, and thought the noise was gone. Then just one day later, the noise came back, not even a bit less, just same old loud whistling noise around 70km/hr and 30km/hr.

What had they done with my car in the past weeks... I don't know. What I know is, I gave them the chance and time to work on it, the result is, they return me a car with the same old problem but claiming they got it done. I felt insulted and am totally pissed with their way of doing things.. If you cannot get it done, just admit you failed, and let us move on... geez, this is MB at its lowest.

THAT IS IT!

I am totally disappointed with MB product, and I am gonna go ask for refund by escalating to HQ or MB Canada President. No, a replacement of C300 will not even do, cuz the new replacement may still have that issue they FAILED to fix... I just had enough with this carp.

Bye Bye Mercedes

One final thought. Have some independent mechanic drive the car and I believe he will immediately tell you it's the differential. That type of whine occurs at a resonant point, frequently in the differential.

I would say that it is probably transmission related if you said it did go away, even if it was temporarily. Sounds like a bearing or a bushing in the trans gets to a point where it starts making noise. Eventually it's going to be up to the engineers to find out what bearing/bushing or whatever is the cause.

I would think that if it was the differential or something else in the drivetrain than replacing the transmission wouldn't have likely remedied the issue, even temporarily.

Sucks that this is happening to you guys though.