I have a C280W 2007, which has been taken to the dealer (2 different ones) to diagnose/correct this problem.

After driving for about 20 mins (more frequently in cold weather) when I apply the brakes and come to a full stop - there is a tapping sound from the rear quarter panel on the passenger side.
It is not a rotational noise - only happens when stationary with the brake applied.

So far they have replaced the whole gas tank assembly - but to no avail...

 

i have the same noise...its like a ticking or tapping noise like something came loose and is vibrating against something back there...hmm

 

In the picture below, look at the part labeled #25. EPC says it's a "valve". I am not sure what this valve is. This may be contributing to the noise.

 

That is shown in my copy of the EPC as a EECS shutoff valve. I'm not quite sure which pipe it shuts off.

Are you quite sure that the noise is not one of the fuel level sensors bobbing up and down as fuel sloshes around in the tank?

 

Unlikely, since it occurs when the car is fully stopped. Actually sounds like tapping from the inside of the panel - when the brake pedal is applied/held. Not in park. Chronic/inconsistently for the last three years.
As I mentioned more during the colder months of the year....

 

Your trunk monkey is not using his seat belt. Problem solved!
Although unlikely, is it possible something is loose in the spare tire well causing it?

 

Anyone?

 

I also have this problem. I fine it to be more of a popping sound coming from the rear.

 

mines like a clanking sound sound taht is elevated when i go over bumps etc...

 

targa, I have this same problem. Not sure if it is from the rear wheel well or the rear more generally (I thought it might be the differential). Happens more frequently in the cold weather, and is not consistent. Sometimes I will hear it more than others. Any help would be much appreciated.

I was planning to have my ATF flushed soon and was hoping that would resolve the problem.

 

These cars all do this. I think we've narrowed it down to harmless fuel level senders sloshing around. Just because the car is not in motion does not mean the fuel is not sloshing around.

 

my noise is more like a metal-metal clanging sound and only happens when i go over bumps etc. and doesn't sound like its anything to do with a fuel floater etc. Sounds more external and the suspension area, etc.

 

Maverik, the tapping sound described by OP is entirely separate issue than your car's issue. OPs car makes that tapping noise AFTER the car comes to FULL STOP. On the other hand, your car makes the noise while the car is in motion (going over the bumps).