The guys at tire and alignment shops are real good at finding the causes of noise. An inspection might be in order.

There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.

I am having the same problem with my 98 Cavalier. I have checked the brakes, suspension, and all mounting and have found nothing. seems that it only pops if I brake from higher speeds and pops as if something is relaxing when I let off the brake and begin to accelerate. I look forward to hearing if you ever locate this problem as I have found nothing that would be causing it.