I agree with William. If the work on updating the timing chain tensioners has not been completed, my best guess is... that is the problem. Before my 1997 XK8 had this work done it sounded like a tappet type noise. The upper tensioner is the most crucial of the two as it can "take out" the motor.
2001 Jaguar S Type 4.0
1997 Jaguar XK8 Convertible