you are better off asking this question in the yj section, since alot of yj's were produced with this motor and not alot of xj's. i have a 91 yj with the 2.5 the diesel sound you are hearing is normal and is the timing chain tensioner. mine has done ever since i bought it in 1999 with 60,000 on the odo. it does sound like a meat grinder though.
this noise is not in the 4.0l because the 4.0l has a different set up for the chain. mike
06 kj 3.7l, 6speed manual, np241 command track
91 yj 2.5l, ax-5, np 231 3 inck lift, 31'' mud rovers
12 kk 3.7L, 42rle, mp1522, d-30, 8.25.