Hmm...

I opened my trunk and listened to it while standing in front of it and it sounded weird. It sounded like it was hitting something. Like a vibrating or flapping sound. Hard to describe the sound it was making.

hmmmm..... It usually sounds wierd from behind, but not like your describing and I don't think xxx's have tensil slap problems check the box see if that's the problem....find out exctly where the sound is comming from.