clunks

I have a 99 Tahoe LS with a 5.7L 4x4 4l60 trans. I start vehicle. Put trans in reverse. At rolling speed after about a foot or two I hear a clunk and feel it in the front end. It drives fine after that initial clunk. Does not happen when starting from a stop while driving. Brake pads...

Hello!! Bought a 2014 Tahoe LS 4WD and runs perfectly fine just what are those clunking sounds underneath at rear sounds like (marble hitting aluminum can, etc.) 10sec intervals right after I shut off the engine? Could it be metal cooling down or is this normal?