I may be ahead of many of you all in miles, I'm at the 73,000 mile mark and mine also started as a thump, it gets worse to the point it sounds like a pop sort of like a bushing popping or the frame popping in the trunk area, especially around the 1-10mph speeds, can be recreated the most while backing up and turning in either direction, it sounds like when the tires are getting filled with air and make that popping sound when they are adjusting to the rim... I can notice it also when getting out of the car it makes the popping sound, we got it up on a lift and everything suspension wise underneath seems fine, you can park the car and push the body around from the back end and push down and sometimes if your lucky it will make the popping sound, so I believe it has to do something with the suspension or frame since the sound can also be recreated with the car off in park and not moving at all, the sound has gotten very very loud now