2007 Chevy Cobalt Thumping

Within 10 minutes of starting my car and driving for a while (always after the car has been sitting for at least a couple of hours) there is a thumping coming from the back right side of the car (4-5 thumps). It has happened both while I am driving and while stopped at a stoplight/stop sign and during summer and winter. The antenna is tight so we have ruled that out. We have even tightened it to make sure. Definitely not the tires too. Almost sounds like it's coming from the trunk or as if someone is knocking on it from the outside. The car is in good shape and the thumping does not seem to affect anything while it happens. My gas mileage is right where it should be (25-30 miles to the gallong) so it does not appear to be affecting that either.

I'm more concerned with making sure it isn't hurting anything in my car. Doesn't bother me other than I can't figure out what would be causing it and neither can friends of mine who have decent knowledge of cars.