Humming/whirring/vibration

[Cottontail]

Ok - first the specs:

• 2001 model year
• 3.5" lift kit with 2" spacer/shackles for 5.5" total
• HP D30 front with 4.10 and Aussie locker
• Fixed aftermarket lowers, OEM uppers
• Drop brackets
• Adjustable trackbar with double shear bracket
• OEM brake set up
• 265/75/16 MTs
• Not running a steering stabilizer or a sway bar
• ZJ solid tie rod
• Checked before my 500 mile road trip last week to be sure all bolts and nuts on the front were tight. They were.

I notice at highway speed, starting around 45 mph that I get a variable humming or whirring noise from the front end (as well as a vibration). It is not constant in pitch, band it increases or decreases in tempo with the change in speed of the truck. It hits a high point in volume, then softens, then gets louder, then softens. The Jeep tracks pretty straight.

The front axle shafts, both of them, had brand new 760 u-joints installed just before the Cherokee Crawl in July. The front drive shaft has all brand new joints. Again, installed in July.

I have pulled the front diff cover to check the locker and check the axle shaft seating in the gears and I see nothing out of place or visibly out of alignment. The fluid was free of metal flakes.

I did notice last time that I was in 4x that I had some noise from the front axle which was what lead me to check the axle guts in the first place. It sounded more like two pieces of sandpaper rubbing against each other than it did metal on metal or clunking or anything else.

So, today I took a little drive down the interstate with the driveshaft off and the vibration and noises were gone. That leads me to wonder:

(a) is there an issue inside the axle that is getting magnified with the drive shaft on
(b) is the driveshaft out of round
(c) is the axle off center and that noise/feeling is due to binding
(d) all of the above
(e) none of the above

I'd like to hear some thoughts, if you have them.

 

[highspeed]

I vote driveshaft. Mine is loose at the slip joint and I get some weird low humming when in 4wd. The fact that the slip joint has play leads me to believe that at highway speed....Sir Isaac Newton takes over and the shaft starts flexing and as it spins...........think of a jump rope as it spins. Seems like if it was the diff, it would still be making noise. Just my .2

 

[md21722]

^^ often its the double cardan joint of the driveshaft. If you can call it a cyclic vibration that comes and goes every second I wouldn't even second guess it.

 

[wavingpine11]

I'm voting a) due to bad pinion bearings, based on your description of the noise, and b) based on the precession of the drive shaft caused by the loose bearings rather than the double cardan joint that highspeed and md21722 have suggested. I just solved this very same noise on my front axle after having it for 5 or so years upon replacing the pinion bearings. If the drive shaft is still off, look for play in the pinion shaft. Mine had slight play both in and out along the pinion axis and also some sideways play. No more sandpaper noise for me. My best description of the noise was that of a snowplow scraping against the road at a distance, but the sandpaper description also sounds good for the noise I was hearing. Plus, if all serviceable u-joints are know good, and you can't see any play in the double cardan joint, it's not likely to be the drive shaft alone. The key diagnostic point for me was that gritty noise that only appeared when the front axle was actively driven, which it isn't if you took the shaft off. If I let my foot off the gas, the noise was gone. The pinion bearings turn faster than the carrier bearings and consequently wear out more quickly. Also, I think the previous owner messed up the preload setting on my axle by replacing the bearings by reusing the old preload shim stack without checking the actual preload. Of course I could just be reading pinion bearing problems into everything since my experience is quite fresh in my mind, but the gritty noise sounds very suspicious.

Alternatively, you could have two interrelated problems, one of which may have helped cause the other, with the bearings and the drive shaft.

 

[natesjeep]

Sounds like what I have going on. I just pulled the front drives haft and still had the problem so I'm thinking it's the pinion bearing like wavingpig 11 suggested.

 

[wavingpine11]

Just be cautious tearing the differential apart. Do your research or you'll easily be in over your head. Check for play in the pinion first, and if there is some, check to see that the pinion nut is at least at 160 ft-lb of torque. There are special tools and a lot of setup work involved in doing something seemingly simple like replacing those bearings. And if the issue is just a loose pinion nut, and the bearings aren't junk yet, tightening up the nut may be the easy way out. If it's not loose and there is play in the system, the bearings are probably junk. Mine were junk.

It would be cheaper to buy a used axle with good bearings than to buy just the tools to do the job right. I like being able to do things myself, so I invest in the tools and do my research when I run into these kinds of problems because I'm guessing that I'll be setting up at least four more axles in the foreseeable future. I'm probably $600 or more deep in special tools for axle work. I bought a rear Ford 8.8 axle for $250. I see Dana 30s going for less. . .

 

[ehall]

which transfer case do you have

 

[trippled]

Sounds like you may have 2 problems: bad driveshaft and a bad inner pinion bearing. FYI I have seen bad bearings with NO metal in the oil. I would try rebuilding or replacing the shaft first and see what happens if you have no play in the pinion as stated above.

 

[Cottontail]

I have the 231. I am going to get the drive shaft looked at first. I rebuilt it with new u-joints back in July, but that doesn't mean it isn't worn out, or out of balance. Start with the cheap stuff and eliminate that first!

 

[natesjeep]

You should be able to barrow most of the tools from oreilys or autozone if it does end up bearing the pinion bearing.