I put mine on the ramp at work with the engine running - sounds like loose stones in a steel pipe, which I guess isn't far off. Its been doing it since last year on and off but got worse to the point of being embarrassing . Passed the mot ok here in the uk but I couldn't live with it any more. Just got to find out the scrap value of the old one now to attempt to recoup some of the £200 it cost (comes in 2 parts one with 3 cats and one with 1 . Needless to say it was the dearer one

Sorry just noticed where you are so you know all about MOT's!