2002 Drivetrain Hum


TJ Enthusiast
Sep 2, 2022
Gilbert, AZ
Hey all, 127k miles on my '02, 4.0L, minor lift, 31s. Had been running great then I noticed a type of 'hum' when coasting or decelerating. Its consistent but there's a pause between each hum. Its not a clunk or a squeak that I've been finding here through search. When I press the clutch or put it in neutral it completely goes away. Lifted up the front wheels to look for play or a wheel bearing issue but all seemed ok. Checked front differential fluid, was a tad low but just a few ounces. Any ideas?
I should add I've been running the RPMs up higher lately as I've read about the 'lugging' thing that I had probably been doing before.
All fluids are up to date.
I'm not sure. Its got a 1" body lift & Jamison said there are 2" spacers on the springs. IDK what stock looks like because its been like this since I bought it 10 years ago. I was thinking of popping the rear differential & just inspecting it. Have synthetic gear oil in there & I see you prefer conventional.
If you still have the stock control arms installed your pinion angle is at the ok factory setting. I was just wondering if someone had lowered the pinion angle which can cause drivetrain vibrations.
Ok good news! Got the rear wheels up off the ground & started rotating & the sounds seems to be the right rear drum brake dragging or something. Dropped the rear differential cover & looks fine to my eyes. So good opportunity to switch to conventional 80w-90.
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First highway trip today, feels really nice and smooth. Still a slight noise that sounds like its from the front but I know sounds can travel around. Doesn't happen while drifting now, only happens when lightly on the gas. Doesn't sound terrible but its there & mostly 30-40 mph. Still would call it an off and on 'hum' every second or two. Any ideas? And it's silent when the clutch is in or in neutral.
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Its not perfect by any means but for 20 years old & 127k miles I think its ok. No big chips or pieces in the gear oil, just fine particles. But this is my first Jeep so I'm not really sure? Have changed the gear oil several times since I've had it. (@ 71k miles)