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Hummm in the front end

98XJSport

Destiny is the rising sun
Location
Western Maine
I have a 98 XJ 4.0 auto with almost 200K. For the past year or so Ive been hearing an oscillating kinda hum in the front end, sounds drivers side to me. Not anything I could feel, so I never worried much about it. Before I took it off the road for a regear/rebuild it was getting louder, enough for me to be concerned. So I replaced the drivers side unit bearing and u-joint. No change. Took it off the road for a few months, put in new trac bar, new tie rod, gears, locker, carrier, all new bearings, new control arms and RC drop brackets. Checked out everything we took off and reused, no signs of wear. Back on the road now and the hum is still there.

I can't tie it to anything in particular, only it does it more at higher speeds. And it seems to change/go away with dips in the road sometimes. The only thing I have seen to check next is the front drive shaft. No play in any of the joints, but the slip shaft feels much looser than the new front driveshaft I am running in the rear. Not loose as Im turning it, but if I push up and down I can kinda rock it. I have to pull my tcase skid to drop the front shaft I think, hoping to try that this weekend.

Can anyone think of something else I should check or have overlooked while Im under it?

Thanks :cheers:
 
Took it for a drive, little bit mroe info. The sound becomes noticable ~45, really annoying and turns into a little vibration 50-65. Can feelit through my feet mostly, tiny bit in the steering wheel. More like a buzz than a steering vibe type of issue. I also notice it doesn't matter if Im accelerating or coasting. And its far less noticable going uphill, gets louder on straights and as the front seems to compress in a dip in the road. I wanted to put in 4x4, but the front locker on dry tar at 60 changed my mind.
 
It would be real easy just to say BEARINGS. But, anytime I've had a bearing go, it's more like a grinding, rubbing, like sharpening the lunchmeat slicer at the deli. Unless, of course, it's the carrier bearings, or possibly the pinion bearing, although that may be a little more dependent upon power on/off. I'm going to go out on a limb and ask if there's any chance at all that it could be chunky-tread tires? Back in the day when those of us in the snow belt used 'snow tires', they usually had a nice howl to them.
 
My 90 has always had a kind of humming...I thought it was just because it didn't know the words...but seriously folks...it's really kind of a drone, that's as good as I can explain it...I am deaf in my left ear so I can't hear the little tics and hums...but there's still kind of a wooo-ooooo-woooo and although it does increase with speed it's not RPM related. I thought it was just the nature of the beast
 
I dont think it's bearings, the only bearing thats not new up front is the passenger side unit bearing. And that one is actually from the drivers side, I took the drivers side out and replaced with new. Then since it didnt go away we put the drivers side used back on the passengers side. All others have been replaced.

Its also made the noise through 3 sets of tires.

Its something like that ParadiseXJ, it's gotten worse since I first heard it and am starting to feel it too so I gotta get it pinned down before I end up miles from nowhere broken down.
 
pull the front driveshaft.

I'd bet it's the front double cardan wanting to be rebuilt. I didn't have too much noise, but definitly a deep vibe you could feel in the floorboard at 55. Sounded like an angry sparrow in the front end at low speeds in 4lo.
 
Pulled the front shaft, no noticable difference at speed.

What else is there? Ball joints and drag link are the only things I haven't touched. Brakes maybe? It doesn't go away when I press them, but gets deeper. Can feel them almost pulsing when Im almost stopped sometimes.

Tis one has been very fusterating, it's the only thing keeping me from driving my jeep as much as I would like.
 
It's most likely not the ball joints nor the tie rod ends.

It sounds like something to do with your brakes. The calipers could be sticking or the rotor could be warped.

Jack up the front end and remove the wheels. Then spin the rotor/axle shafts by hand. Also make sure that the calupers are bolted down and that they are not loose.


E
 
I was thinking the same thing, will have to check that this week.

Ive always heard that speed sensitive sounds are drivetrain related. And since it's not the front driveshaft, differential or axle shafts Im going to have to look a little further out. Looks like brakes are next.
 
it might not be the front at all, the unitbody transmits vibes all funny. Try pulling the rear shaft. It could be the joint on the rear shaft, and when you step on the brakes and unload the rear suspension the noise changes with the u-joint angle.

What kind of frequency is the hum at? usually lower pitches are driveshaft/t-case related, and higher pitches are something at the wheel/axle.

It's also possible that it's something in the front pumpkin. Only way to diagnose that is to remove the inner halfshafts and the front DS. Don't run the unit bearings without the stubs. I keep a spare stub around just for these types of things.
 
I can hear the hum at a low pitch around 30 sometimes, all the way up to a fast pitch/frequency between 50 and 60. Ill have to try without the rear driveshaft too, Ive got to measure the angles again anyways.

All gears and bearings are new in the pumpkin, it did this before the rebuild too.
 
I've got a similar noise to this description as well...checked out everything except have the same "rock" to the slip joint in the front DS. Also, the oscillation seems to quiet when placed into 4wd making me think it is something driveline related. Thought I would add to the discussion in hopes of a solution.
 
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