My ST165 has started to make a wierd rattle from somewhere near the top of the engine - possibly head/piston related. It seems that whenever I'm just opening the throttle (like maybe 1/4) it will rattle *very* audibly, a full on metal-on-metal knocking sound. What the hell could this be? It doesn't seem to rattle under other loads (well it may be inaudible at lower load, and maybe the cylinder pressure would be causing it to *not* rattle at higher engine load).