I have a 2000 XJ with the 4.0 and about 200k. I bought it a few months ago and it had the usual tap/knock when cold. Yesterday on the way home from work I noticed a much louder knock. I stopped and checked the oil level, it was a quart low so I put a quart of Lucas in it. The knock did not change. After some research I pulled the valve cover and couldn't find any problems. Next I removed the dust cover from the bell housing and checked all the bolts on the flywheel, all were tight and the flywheel looks ok. The noise seems louder under the Jeep and more on the driver's side. The oil pressure stays above 40 and isn't dropping or fluctuating. Any ideas would be greatly appreciated.

 

The flex plate may have cracked around the crank shaft bolts.
It's very hard to see unless you pull the transmission.

I would block the wheels and set the e-brake and crawl under it with a scope to pin point the noise first.

 

I got under it with a scope and there is a rubbing sound from the oil pan. There is also a "tink, tink, tink from the exhaust manifold coming from the #5 cylinder. I can also hear the knock from the bell housing.