I have a early 2004, January 1...4.2 XKR .On cold start up there is a rattle on the back top of engine. It only lasts 3-4 sec and less if I engage gear as soon as starting .

When warm no noise at all REMEMBER its a 4.2 XKR. any ideas what it could be, or am I worrying about nothing?
My gut feeling is Supercharger

 

My gut feels the same!

Unless you aren't using a Jaguar oil filter or an equivalent with a non-return valve and it's taking those few seconds to build up pressure and lubricate the cams.

Graham