Originally Posted by
Al1943
This sort of thump is often associated with the rim joint, sometimes cured with a flat file.
+1
You'll also probably see some rubber transfer on one side or the other of whatever is causing the thump. Look for the Telltale black (or salmon) transfer and feel that area for any distortion.
You might also wait until it happens, then get off the bike and spin the wheel with the brakes closed just enough to work as feeler gauges, and possibly see the exact cause.