I have just recently fixed a knocking sound in my 91 xj 4.0. It turned out to be loose torque converter bolts...veryyyyy happy. It sounds quite like a diesel now for some reason. Not as loud, but a subtle diesel sound. Is this normal? It sounds kinda like maybe exhaust is leaking from the manifold or actual pipes or maybe loose rocker arms or valves? I don't know. The sound is loudest when I am underneath her but is still very distinct at the top of the engine towards the valve cover, Any input would help.

Chances are a leaky exhuast manifold. The 4.0L's are kinda known for this. The 4.0L in my *cough* '98 Grand *cough* *cough* is has had one for about 7000 miles now.

^dude, you need some tussin or something?^

x2 on checking the manifold by SirWJ/ZJ up there. Their notorious.