My '96 XJR has that noise on startup would say it last for about 3-4 seconds and then goes away. Slack on the timing chain? How is this resolved?

 

Please find and read all TSBs for your car as they will answer most of the questions/problems related to these cars.

 

The uprated tensioner can be obtained from a salvage yard

It has a large arrow cast on the body

get the bolts as they are different grip length

Be careful to not to drop it through the engine compartment , easily breaks the tensioners inner workings

Ask me how I know

Installation covered here

Installing (revised) upper chain tensioner - Jaguar Forums - Jaguar Enthusiasts Forum

More info here in the Doc below

See page 29 through 31

The piston / engine block has a large O - ring Jaguar part # EAC4540 , you can see it as the black ring in pic