Yesterday I started the xj and I got a rattling or knock that sounded like either a lifter failed to pump up or a valve was getting tapped by a piston crown.

It did this for a minute or two...then suddenly stopped and the motor sounded normal.

Any ideas what this might be??

 

Sounds like a lifter leak down to me. It happens. If it continues to do it you may have a lifter failing.

 

Bad/stuck lifter. Add a quart of Marvel Mystery Oil or ATF4 to the crank case. Oil change after 500 or so miles.