Clicking at start-up

Gents,

I've posted this here as well as in the XG forum since they share the same 3.5 Sigma. My vehicle is a 2004 XG350L with 120,900.

I've got a clicking sound at start-up, especially in the cold, coming from the cam cover. It goes away after the engine is running for a while. What could possibly be causing this? How do I fix it?

Thanks.