Rear diff or transmission hum between 40 and 55 mph

[bob850]

1993 Suburban C1500, 5.7L, 2WD, 3.73 G80 rear diff, auto tranny (4L60E), 135K miles.

There is a distinct whining noise between about 40 and 55 mph when load is on the drivetrain. If I take my foot off the accelerator and coast, the noise goes away. Again, it starts around 40 mph and goes away completely around 55 mph even when under load. It sounds like its coming from the front, but I can't really tell. The noise just sort of fills the whole cabin.

The transmission and rear diff fluid were both recently changed, and have been changed on a regular basis throughout the life of the vehicle. The most recent fluid changes had no effect on the sound. I think the noise has been present for a number of years, but seems to be a little more noticeable over the last year or so. The drivetrain has never been abused, but is getting up in miles.

I did some searching but can't find a good answer on the specific cause. Any advice from the experts on how to fix? I posted this on another forum a while back before I discovered this gmt400-specific forum (they suggested u-joints, which have never been replaced).

 

[bob850]

Has anyone else experienced this?

 

[Tech-GeeK1500]

From my experience U-joints tend to make more of a clunking noise when shifting from drive to reverse or reverse to drive, while the truck is sitting. The opposite and immediate load placed on the u joint causes the excessive play to shift the joint back and forth. A groaning or humming/howling noise is usually associated with a bearing somewhere in the drive line. Double check your transmission and rear diff fluid, also be sure to check the fluid level in your transfer case.

 

[bob850]

Thanks. Vehicle is 2WD, so no transfer case. Fluids are good. Are you saying it might be my U-joints? My front wheels bearings have been replaced recently (with front brake job). However, my U-joints and rear axle components are original (135K miles on vehicle).

 

[Tech-GeeK1500]

There is a possibility it could be a u joint, but more than likely not. I'd check the pinion bearings in the differential. It sounds like those are suspect.