Please pay attention to the original post, he has a K100D, not a K10D.

I feel the same on my K100D, and a GX-10 as well. It seems like a heavy part on the inside is sliding, I'd be surprised if that is really the sensor plate - should need some very strong magnets to keep it floating during a shot.

I've just sent an e-mail to Pentax UK asking about this. I feel better knowing that it's not just my camera (may be a design "feature", but it's better to ask