I have a 2000 4.0L Jeep. It's developed an engine noise that is not a clicking noise you'd expect from a noisy lifter. It sound like it's coming from the lower part of the engine. It does not sound like a rapping noise you'd expect from a rod. It somewhere in between. It's not a loud noise. The noise usually is apparent after the engine is first started or starting the engine after its been off for awhile. After I start to drive for a couple minutes the sound goes away. Oil pump? If I accelerate the noise goes away. Under engine load there is no noise. Any suggestions? Thanks