Topic: Humming Noise When I Let Off the Gas

[DanMarino]

Hi Everyone,

I have a 1994 YJ with the 2.5 engine and AX-5 tranny.  I noticed a humming noise today when I would let off the gas while driving.  When I was on the gas, no humming.  It hummed when I was both in gear and in neutral.
Any ideas what I should be looking for?

Wheel bearings?
Some other bearing someplace?
Rear differential?
Driveshaft U joint?

Thanks for the help.

13

[Jeffy]

Could be tires even.  You're going to have to localize the sound.  Best to drive around without the top and doors.

[DanMarino]

Since I am deaf in one ear, I can't locate noises.  So I'll have my son or wife give it a listen.  In the mean time I'll check out the driveshaft u-joints as a start.

[Jeffy]

When you say in neutral, are you still moving?

[DanMarino]

Hi Jeffy,

Yes, I am driving the jeep.  Doesn't matter what gear I am in or if I am coasting in neutral.  If I'm in gear, and have my foot on the gas pedal there is no humming sound.  As soon as I let off the gas, there is a humming sound that continues until I give it gas again.  When I stop at a traffic light, there is no humming sound.  So I've got to be moving.

13

[DanMarino]

I'll take a look at the engine and transmission mounts too.

[Jeffy]

So noise while under power only?  If it was more like a can of ball bearings sound I'd suspect the driveshaft U-joints.

[DanMarino]

I'll drive it again tonight and take a look underneath. 
I must not be explaining it well.  When I am driving the vehicle and let off the gas pedal, then I hear the humming sound.  I'm not sure if it is a vibration or a hum, or where it is coming from.  I'll start a process of eliminating components and report back.
Thanks again.

[DanMarino]

Update. - Jacked up the passenger side of the jeep last night and did some investigation.

1.  I didn't find any play in the drive shaft u joints.  Front and rear u joints seemed fine.

2.  I tried to wobble the front wheel up and down, left and right to see if there was any play.  All seemed tight.  Wheel bearings seemed fine.

3.  I checked around the exhaust system to see if it was bumping or rubbing anyplace and all seemed fine there.

4.  I did notice some play in the transfer case rear tailshaft.  there was movement up and down or left and right when I applied pressure there.  It moved maybe 1/8 inch or a little less.  How difficult is this seal/bushing to replace there?  Is it normal to have play at that point?



Thanks,



13

[Jeffy]

How much lift do you have? How much of the slip yoke is exposed?  Have you ever replaced the U-joints?  I suggest greasing the upper U-joints and see if it goes away.  I highly doubt replacing the rear seal will do anything.  Play is caused by wearing on the output shaft splines and the drive shaft slip yoke.  This us usually exacerbated by bad driveshaft angles.

[DanMarino]

Thanks Jeffy. 
I am stock on flat springs from 1994.  haha.  No lift for me.  So the angle is not severe at all.
I'm pretty sure I have had a shop replace one bad u joint before.  I'll take a look at my maintenance records.  I'll also look and see if my u joints have grease fittings.
13

[DanMarino]

It sounds a little like this, but only when I let off the gas.  https://www.youtube.com/watch?v=EjoaGOrhQco

From what I have researched, it could be that the pinion nut needs to be tightened slightly.  Apparently as things wear, there gets to be some slop in there and the angle changes slightly when the driveshaft is under load/not under load.

[Jeffy]

If it's your pinion yoke you should be able to shake it and it would move.  Pinion side shouldn't move.  U-Joints typically sound more gravely though.  I'd grease the U-joint and see if it changes.

That video sounds like tires or a worn pinion bearing.  Smooth hum.

[DanMarino]

For the heck of it I changed the transfer case fluid.  Things were fine there.
Next I took the cover off of the rear differential.  Metal flakes in the fluid as it drained out.
Found this.

[DanMarino]

Each of the 2 small spider gears have lost a tooth.


It sounds like the Dana 35's are notorious for exploding.  People hate them.
I don't wheel this vehicle.  It tows my kayak/utility trailer and gets me around in the snow. It sits in my garage probably 5 or 6 days a week.

The pinion gear teeth and ring gear teeth look fine.  I'm not sure what else to check.