What you are hearing is most likely the plastic on the door flexing and popping back out. I had that problem with my old box before I went with the kicker L5 that took up less space. The sound echos through the spare tire space and sounds like one heck of a clunk. I thought my rear end was going at first because when ever I would accelerate or go over a bump the box put pressure on the plasic it would make the noise. You can try putting some bracing between the spare and the plastic to make it more ridged, but the best solution would be to build a box sized for your VX that doesn't rub. Hope this helps.