I have similar issue with K-x. After shutter actuation, and when mirror is already back in place, it makes sound as if sensor inside is moving. It's not tied directly to shutter mechanism, since it doesn't do this when coming out of live view (and it doesn't do it during taking picture in LV either, but then SR is probably on the whole time), but otherwise it does it after every single shot (not during sequence). I can't tell if it was doing it since I bought, but quite possibly, since it's not that very audible and I could have missed it.
Well, anyway, after being terribly worried about it for about two days, I just decided to ignore it. I am pretty certain it doesn't affect camera operation in any way. That said, I'd still like to know if this is commonplace or not.
BTW, one guy at DPR forum suggested it may be film advance motor Pentax designer forgot inside.