I just looked at a 91 Legacy turbo with 138,000 miles on it. It clacks badly on the top of one of the heads.

My question is, does this use solid lifters with shims to set the clearance? If so, it sounds as if one of the shims fell out of the lifter body.

Anyone have any opinions on this?

normal problem, your lifters are bad. You can go to a junkyard and pick like 30 or so lifters up for a decent price, and take those along with your stock ones and test to see which are the best. Then just put em back on. It only takes a few mins to replace lifters I believe. At anyrate come visit us at http://bbs.legacycentral.org