Annoying Noise in C-Class

Hi,

I have a 1995 C280 that was recently repaired and a whole new right rear door was put on. I started the car up one day, and this repeating mechanical noise kept occuring in the right rear area of the car where the fuel filler door is. Its hard to describe the sound, but it sounds like a mechanical noise of something being filled up or simply a repititous mechanical noise without much delay between repititions. It don't hear it with the car in park or neutral, but it only occurs in drive or reverse gears. Also, after I've been driving a while, it appears to go away after about 5 mintues. I also drove a 1994 C220 a while back that got the same problem around 75,000 miles which is how many miles are on my car. Does anyone know what is causing this noise? I figured it might be a fuel issue or a short in the electrical system.

Anyhow, I would appreciate any reply to this issue.

Thanks,
Troy