I get a definite "clunk" or "thud" sound about 10-15 seconds after coming to a complete stop on off ramp after extended highway travel at 70-75 MPH. It is definitely coming from back of car and sounds to be coming from the roof (altho hard to pin specific backend location down).
It does not happen with stop and go traffic speeds from neighborhood on up to
~ 50 MPH or more.

I've looked and nothing is loose. Has been like this for at least a yr w/o change.

I recently had a flat, removed space saver/jack etc and haven't put it all back yet as haven't figured out the "puzzle" of how to get it all back in there just right so it fits. I thought the sound was more prominent with all this sitting loose in the back, but with further close attention, can not definitely say that is the case.

Its as if something was on a pendulum, then came back to a stationary position after the complete stop, causing the sound as it returned to "baseline" position. (Best summary description I can give)

Any thoughts on what this might be?

Sam 2001 V70 T5 90K miles

(BTW, my ETM went at ~ 60K)