Sun roof rattle - metallic

Ok so since I've owned this lovely car I've always had an annoying rattle from the overhead compartment where the sunroof sits. 

Thought I'd try driving it without any of the sunroof components - runners and sunroof removed - and it still bloody makes the noise!!!

Definitely too metallic for wiring to the aerial to be the cause. Can't see any issue within the sunroof compartment when I look in. 

Anyone shed any light??!! Mysterious!