Timing chain adjuster

I'm not sure what model your Jag is but, for the XL8+ there is a TSB/Recalls (Technical Service Bulletin) for:
NOISE DURING START UP - INSTALL REVISED TIMING CHAIN TENSIONERS:
Some vehicles within the above VIN range may experience a rattling noise when the engine is started. This noise may be caused by the secondary timing chain tensioners. Research has shown that a noise can originate at the chain tensioners to cause a resonance in another part of the engine.
A revised, spring assisted, secondary timing chain tensioner has been introduced to eliminate this concern, commencing at engine No. 98102106XX. Introductory VINs are not yet available. In case of a customer complaint, the revised tensioners may be installed on earlier V8 engines.
But then again, I'm not sure exactly of your model, if you get back to me with the exact model, I can check for any TSB/Recalls for you :)