OK - Looks like it's my turn. Sounds like something inside the EP500 has started to rattle on some scenes (for me the new Planet Earth Blu-ray) and it is not calibrated hot (or it wasn't). Looks like from this thread in order I should try:
1) Tighten back screws
2) Tighten front scres
3) Double check some setting is not running
4) Call Axiom!

Thanks
Nathan

HT:M80,VP150,QS8,EP500 Outside:4xM3 Office:AudioBytes, Rumpus: M60,VP150,M22 Portable:2 x Airs