Hey mate that could be your brain rattling...are you sure it's the cap  if it is the cap try holding the plastic bit and twist tighten the chrome bit.

I was gonna go a rubber band temporary idea... or even blue-tac

Hey Stix ya should get yourself a Softy mate..........Smooth as silk and not a hint of a rattle even with a loose cap

But then again it might keep ya awake on the Nullabor

Bt