I have a stock Xj that has recently started making a rattling noise at idle. I took apart the bottomed end and found nothing loose or worn and did the rod bearings and rear main seal while I was there. Any ideas?

 

Buy a stethocope. You can also use a rubber hose, or a long screwdriver by putting the handle to your ear, and other end on whatever it is you want to listen to.

Describe the rattle. Is it up top, or underneath? Does it sound hollow? Metallic? Does it change with rpm? Does it go away once the engine warms up? We have no clue what youre hearing.