thumping from passenger-side rear

Hello again. I had the strangest thing happen today. I started my 2004 TB EXT at the auto parts store (getting brake pads for my grand am), and a strange thumping started coming from the pass-side rear. I got out and popped the hatch. Sounded like it was coming from trim panel under rear window. I held my boot against the tailpipe, thinking maybe I broke a muffler mount; didn't help. Figured I'd try getting it home before something let loose. Sound went away about 100 ft down the road. I'm guessing it was something electrical. Anyone have any input? Thanks.

Could've been debris on the tire or the hatch not fully engaged. But in the end, if it went away, it's all good.