Nitro Rear Suspension

My Nitro has just started to make a noise like a piece of loose suspension is thumping the underside. Just started 2 days ago. I tapped everything underneat with a rubber mallet, took stuff out of the back tray, slammed all the doors, checked the spare and jack for looseness, visually looked to see if anything was loose, folded the seats down and it still does it! The only thing I have done before it started was fill up with gas. Anybody with ideas or had similar problems?