I have a noise coming from behind the seats on my 2.8, possibly from the roll hoop-subwoofer-seat belt tensioner enclosure. Its a dull 'thunk', not necessarily something loose but two things too close to each other. On rough roads I get a stuttering "thunk-thunk-thunk, thunk-thunk" corresponding to the road surface. When the car hits a major bump, there is a single "THUNK!", considerably louder than the stuttering thunks. Just to make it more interesting, this seems to happen only when the top is up. I've done a cursory inspection of the top frame and found nothing loose, but I could be wrong. Anyone have a similar gremlin or able to offer a fix? Thanks