Noise from rear

My car did not run for about two weeks, before I picked it up from the shop where they changed the harness that goes to the fuel pump.
Funny enough, now I hear a loud thumping noise from the rear, may be more from the driver's side. It gets faster as the car's speed increase and slower in frequency as the speed decreases. To give you an idea, at 20mph it thumps about twice a second. One can barely hear it when I press the gas pedal, but if I take my foot off, it gets much louder and it is not just because the engine isn't as loud...
Well, recently I changed the fluid in the differentials, I don't think I have the limited slip, but went to the stealership and bought the modifier...
Also, I had to adjust the rear brake on the driver's side so the car could stop.
This is all the info I can give you. I have no clue, as just by looking I can't see anything unusual.
Thanks in advance
Boris

Glad that I got so many suggestions...
Just in case anyone wanted to know, the only thing that came from my search on this site before I asked was the overtorque of the lug nuts (which I did not really believed to be the reason) and I went and checked...guess what? They were soooo loose...the idiots at the shop did not tighten them up and the wheel was making the noise.