Howdy,

On my 1988 4Runner Auto 4x4 Ive had a strange light groaning noise for a few months. I believe ive narrowed it down to the rear portion of the truck, passengers agree it sounds as if it is from the rear.

Here is an outline of the noise and what ive checked so far. It is important to note that when i got the vehicle the rear axle had 2" lift blocks with incorrect round u bolts, one side was very loose and the lower shock mount/u bolt bracket was rattling around for who knows how long. The e brake cable was also pulled tight over the leaf springs which i have corrected with relocation brackets.

The noise is an intermittent muffled groan that only appears around 25 mph and above. It does not change with speed and the groan only appears to occurr when the suspension flexes lightly. If I am driving over rough terrain the noise is non-existent. It is not consistent with a wheel bearing hum and I have no noticeable driveline vibrations. I recently had the transmission and 4x4 totally rebuilt by a very reputable garage and that did not solve the noise nor did it change in any way. They also did not observe anything out of the ordinary that could be the cause.

- Lift blocks replaced and have proper u bolts, torqued properly in sequence
- Shock mounts torqued and shocks themsleves extend on their own when detached
- Driveshaft and u joints are tight and I greased every joint front and rear
- Brakes front and rear are in good shape, no pulling, softness, good stopping force, pedal is firm
- Installed new dif breather, no diff or axle seal leaks that I can see
- Leaf spring hardware all snug and no unusual movement observed when shaking and bouncing the truck
- Listened to and watched driveshaft and rear hubs spin while rear jacked up, wheels off, and in neutral and in gear, almost no noise at all from any components. Hubs spin normal with minimal effort and only the slight and quiet hissing can be heard.

Any ideas are welcome, ive searched extensively here and other forums and found nothing that matched my scenario.

Thanks!

 

Two random thoughts.
1) rotate tires from front to back and see if noise moves. Even if you're sure the tires aren't the culprit try it anyway.( this has isolated more noises than any other procedure over my years)
2) lift the back of the vehicle by the frame so the suspension drops a bit. then lube between the individual leaf springs.

 

Saw this on a truck rear end noise website.
Noise: Low-pitch rumble at all speeds over 20 MPH
Cause: Worn carrier bearings

 

I will give them a try. Thank you. Tires are brand new 32x11.50 Coopers but anything is possible. Assuming spray type silicone lubricant will suffice?

 

We posted posted at the exact same time go figure. Thank you I will give that a read!

 

In the course of the restoration of this truck I had done all fluids as well as flushes except differential. Rear diff was nasty and it had a fair amount of seperated water in it, prior to this the breather i replaced was gunked up, my guess is someone drove through water a few times while abusing this thing off road. Fresh synthetic gear oil and the noise seems to be greatly reduced. My suspicion is now within the 3rd member. Maybe too long without proper lubrication? No metal flakes in the old oil.

 

Well, if you can safely get both wheels off the ground and run it in gear maybe you can track it down with an automotive stethoscope.
The bearings in the third member under the most stress are the pinion bearings. They have to carry the brunt of the hard work. The carrier bearing just support the axle. They don’t have to work nearly as hard.
you can usually tell if the pinion bearings have issues by leaking at the output seal where the driveshaft flange is, combined with lateral movement of the flange where it passes through the seal.(I.e. play)
my diffs we’re a bit loud with regular 80-90 oil. Redline oil made a noticeable difference

 

Check all the bearings.