When I first lowered my car I thought I had the nuts on the top of the struts tightened, but after I got it off the hoist, I noticed that every so often I would hear this "pop" from the rear when I would hit a substantial bump.

Check the nut on top of the strut. Once I tightened that thing up I haven't had any problems since. What happened was that there was some play in the strut & the "pop" was the struts threads rubbing the inside of the mounting hole on the inside of the car. You wouldn't notice the play while the car was on the hoist because the strut doesn't have any weight on it & the nut will appear fine. Don't know if this will help, but at least check it.