I have a 2002 V70. When I put the car in reverse, and begin to roll backwards, I get a loud clunk and vibration from the rear. It seems to be coming from one or both rear wheel areas, and only for a second or two when I first begin to roll back. Could it be a brake sticking or something more serious? Your advice is greatly appreciated!

 

I would say possibly the E-brake shoes sticking.

 

My 98 does that from time to time, particularly after a heavy rain, if the car has been idle for a day or two.
It's pretty loud, kind of startles you.