My 1986 xj6 has developed a rattle . Especially at idle . Less noticeably in gear .kinda like its a low rpm thing. I crawled under it and it seems like it comes from harmonic balancer. Any ideas???

 

Rattles in neutral, less in gear are usually a cracked flex plate or loose flex plate bolts.

 

Many things come to mind, some I have found over the years are :

Torque converter rattle from the internal thrust bearing/s.
Flex plate, as already mentioned.
Cat converters (recently on an XK8, after trans diagnosed as the cause, and found when exhaust removed for trans access, LUCKY MAN).
Heat shields, all over the place, and they do make weird rattles.

If you think its the harmonic pulley, remove the belts and using a large lever see if it moves excessively. The 6cyl pulley is not a real common failure item, but I suppose age is against it now.

 

....or the rivets holding the flex plate sections together have come loose.

With a hammer and block, some contortions, and luck they can sometimes be snugged down again.

This is known as "banging your rivets" :-)

Cheers
DD