STOP! It's probably (%99 sure) your harmonic balancer pulley is seperating. Dorman makes the replacement. Should replace the front oil-seal in the front cover while you are in there. Rent the balancer puller from the parts store. Can be done in the parking lot if you are in a hurry. No need to drain anything either. Heating the new balancer hole with a propane torch makes it jump right on the crank when re-assembling.

Make SURE you use the puller correctly. I boogered up the first few threads on my old XJ and had to re-tap and use a new bolt. The crank snout has a very soft core.