bvonseg

Hey guys,

I just recently bought another 98 xj cherokee with 200,000 that needed a little work. To get things start the last guy rigged up a lot of the suspension which was awful, so I just replaced all of the bad parts. The problem I can not figure out is this humming noise I get when I accelerate. It's only when I push the pedal down at any speed. Taking my foot off the pedal while driving or putting it into neutral makes the humming go away. I just replaced the rear axle u joints, changed the transmission fluid, and the rear diff fluids. The rear diff fluid was pretty nasty there were some micro silver flakes in the fluid, but after inspecting the gears I couldn't find any obvious chips or abnormal wear. The wheel bearings are all good, and it doesn't have mud tires on it. It does need a track bar seeing as the last guy did not put it on when he lifted it. I have one on the way. Thanks for all your help!

salad

Rear axle U joints? You mean the drive shaft?

Which transfer case does it have? It's in 2H, right?

bvonseg

Yeah sorry about that. The rear drive shaft U joints. Im not sure which transfer case it has. What do you mean by 2H?

salad

Well, by the shifter for the transfer case, it should say 2H, 4H, N, and 4L if it's an NP231 CommandTrac. NP242 SelecTrac has 2WD, 4WD Full Time and 4WD Part time, Neutral, and 4LO.

bvonseg

Thats what I thought you ment. Yeah its in 2H. I was talking to a buddy of mine and he said it might be the rear ring and pinion, or the rear axle might need to be shimmed. I took a picture of the pinion but I cant tell if it has a lot or a little wear on it seeing as I dont know whats good or bad.

salad

So it's lifted?

bvonseg

yeah it has a 3.5 rough country lift on it. lower control arms, transfer case drop, full leaf pack, leaf pack drop brackets, sway bay drop, track bar, springs, shocks. It doesnt have a slip yoke.

salad

Ah. I'd start by looking at an SYE. Shims are really for correcting pinion angle once you have an SYE on there. I couldn't find my favorite picture but this should explain what's going on:



Of course our Jeeps have the slip yoke on the output shaft and a solid rear DS, but that doesn't matter for the purposes of this picture. U joint operating angles are the important bit.

bvonseg

alright thank you I will take a look into that.

Calebp

Hey,
I have the EXACT same issue going on. I am new to jeeps. I installed a 3.5 rubicon express lift - full rear leaf, have a CV driveshaft, and shimmed the rear axle. No transfer case drop. I was wondering if you could post the findings you had with your noise! Thanks