Honda crx noises

I have a 1987 CRX (117000 miles) that has developed an irregular thumping noise. It seems to be coming from the rear suspension but no one has found anything wrong with it. I replaced the rear shocks six months ago but the noise did not start until last month. The noise is not related to speed but does happen more often on rougher roads. Any ideas about this noise?

Have you tried rotating the tires?

The tires are new and as noted in my question, the noise is not related to vehicle speed. The noise is the same when the car is going at a walking pace.