Pontiac: 97 Grand Prix -- "thunk" noise coming from right rear wheel area?

Hoping someone has some ideas on this..

When going over bumps in the road, my wife's 97 GP makes a "thunk"
sound near the right rear wheel. We've had the car in at two
different places (one private, one AAA) and neither one found
anything. The private shop replaced the sway bar linkage (didn't
help) and the AAA shop said the shock looked fine, was tight and not
leaking. Said it certainly wasn't a safety concern, so if we could
live with the noise...

The private shop did mention that part of the honeycomb from the cat
converter was loose, but I have a hard time believing that's what this
noise is. Seems to me a broken piece of honeycomb would make more of
a rattle noise than a thunk, and probably wouldn't be very audible
from inside the car.. Am I wrong on this? The car just passed
emissions inspection, so it must not be a very big piece..

Any ideas what else might be causing this noise? (And no, there's
nothing in the trunk.)

Thanks,
-Wrench

I have a 93 grand prix and I have had the same trouble. I noticed that one of my exhaust pipes was hanging a little low and rigged it up to where it wouldn't move anymore. This solved my problem, and it may solve yours as long as your pipes are secure enough.