I had the EXACT same problem. I dont think they are stripped. I think its just so darn hard to get them in there right it wont take hold. Funny thing is right after i re-installed my resonator, later on that night i was in an accident which killed my whole front bumper and my resonator(which they replaced). So i dont know how the heck they got those bolts started. All i know is i'm not taking it out again!

And yes, that noise is annoying if there's no HP to back it up. My car is so much more quiet from the inside now!

Dave