2001 Jeep Cherokee; 126K miles. I'm hearing a sort of whine or whirr that's coming from the back end of the Jeep when it's under power. As soon as I let off the accelerator, it's gone. It's a very low level but I can definitely hear it. I notoiced it right after I swapped out the rear diff. fluid last month. (Sucked out 2 qts. of dino 75W-90 and replaced with Syntec 75W-90.) But I'm not sure that has anything to do with it. I was listening more intently than I have before when I heard it. It may have always been there. The other thing worth mentioning is that my mechanic did some work on the axle a couple years ago because he heard a noise back there. A bearing I believe. I didn't get a chance to hear that noise so I can't compare. I'd appreciate any expert opinions. Thanks.