"TINK" Noise when I put car into reverse?

[PILFILR]

My... 2003 C2 Tiptronic with 17,200 miles is making a "Tink" noise. I put the car into reverse and start to back up and I hear "Tink". The sound is outside the car near the rear wheels and not muffled. It sounds like you are cracking a lug nut loose. If I reverse real slow and apply no gas, I get no "tink". If I "gun" it...I sometimes get "tink" "tink" (2 tinks). I have hand tightened all lug nuts. Maybe a drive shaft is coming loose?? Any ideas?? Thanks for the help. :thankyou:

[wvicary]

Brake pads shifting? It's difficult to pin point these noises over the net but I would inspect the brakes.