So for quite a while now my xj has made a squeaking or chirping noise for the first few minutes its driven, only when completely cold--having sat for at least several hours. The strangest part is that the noise shows up with the gas applied, and then will go away temporarily if I take my foot off the gas, after a few seconds it will start up again and continue until I apply the gas, at which point it will temporarily go away again. It does not happen with the rear drive shaft out.

The way the symptoms manifest leads me to think that the noise is somewhere in the driveline after the transmission (AW4), the noise does not seem to be associated with RPM. I recently replaced the centering ball and seal on my rear drive shaft (actually a front shaft) which seems to have had no effect.

Any ideas on what else it could be? I'm sort of thinking that maybe the output bearing on my tcase (NP242) could be going bad? Maybe u joints? Rear axle is an 8.8 out of an Explorer fwiw.

 

Probably u joints. Wiggle the drive shaft firmly with your hands and if there is any play (at the joint, it should be solid there) then you've found your problem

 

x2

 

There's no play in the u joints, however when I pulled the outer one on he double cardan so I could replace the centering ball the grease inside the caps smelled pretty cooked.