My 2003 Buick Park Avenue, 66K miles, has developed an odd occasional thump or clunk when driving over rough patches of road (and believe me, we have far too many of those in The Swamp). The sound seems to come from the right rear of the car. I had my regular mechanic look at it, and while he admits he can hear it, he couldn't find anything when he put the car up on the hoist. Everything was tight, including the first suspect, the muffler/tailpipe assembly. Yesterday I checked the lugs on the right rear wheel and found them tight. There's a hard plastic baffle that separates the trunk from the rear seat, and it seems to have cracked and is a little loose (no idea how long that's been going on). I taped it up and jammed the heavy bottle jack case up against it, but I still heard the noise this morning. Nothing else in the trunk seems loose. It doesn't sound metallic, and it's not a rattle. On the rare smooth road the car drives fine at all speeds. Go over a speed bump with the left wheels only, no problem; right wheels, and I hear it. Some bumps elicit the noise, but ripple bumps certainly do, as if the hopping of the wheel over the ridges is causing the problem. But if the wheel was out of balance, wouldn't I notice it on smooth surfaces too? Would a loose rear brake caliper cause such a thing? I had the front brakes done in late '07, and the rear pads replaced earlier this year, but the noise didn't pop up until recently. Any ideas of what I could check?