Confusing hum between 25-50 mph?

[water4543]

Hello, newbie here, so I'm not sure where to post this. I was recently on a 400 mile trip through the desert when about halfway through I began hearing a hum. It correlates to the speed of the vehicle and is only noticeable from about 25-50mph, but is most prominent from 30-40mph. The amplitude (volume) of the hum is the same whether I am accelerating, coasting, in neutral, and deceleration, and the pitch changes according to vehicle speed, not engine RPMs.

I read up on the symptoms of bad wheel bearings/u-joints and the symptoms don't seem to match, and when I checked those parts, they seemed to look fine. Took it to a shop today, and even they couldn't figure out the source of the hum.

I am now quite confused as to what it could be. Any ideas? Thanks in advance.

The truck is a 2010 Ram 1500 SLT RWD 4.7L.

 

[Black W900]

Might have a u joint going bad...My '08 was doing that and it turned out to be a wheel bearing after all.

 

[water4543]

When you initially looked at your wheel bearings, did they have play in them? I checked mine when I rotated my tires this morning and there was no play.

 

[averageguy]

Tire out of balance or wearing unevenly?

 

[chrisbh17]

Yeah maybe tire(s)....try swapping fronts to backs and see if it changes?

 

[water4543]

Tires look even to me. I don't think they're out of balance because there isn't significant vibration while driving.

 

[water4543]

Yeah maybe tire(s)....try swapping fronts to backs and see if it changes?

I rotated the tires this morning and there was no change in the sound or its location. I checked for play in the wheel bearings when I rotated them, and there was no play. Can an axle bearing be bad and only make noise (no play)? Could bad carrier bearings in the diff make a similar sound?

 

[chrisbh17]

GEAR NOISE

Axle gear noise can be caused by insufficient lubricant, incorrect backlash, tooth contact, worn/damaged gears or the carrier housing not having the proper offset and squareness.

Gear noise usually happens at a specific speed range. The noise can also occur during a specific type of driving condition. These conditions are acceleration, deceleration, coast, or constant load.

When road testing, first warm-up the axle fluid by driving the vehicle at least 5 miles and then accelerate the vehicle to the speed range where the noise is the greatest. Shift out-of-gear and coast through the peak-noise range. If the noise stops or changes greatly check for:

- Insufficient lubricant.

- Incorrect ring gear backlash.

- Gear damage.

Differential side gears and pinions can be checked by turning the vehicle. The side gears are loaded during turns. They usually do not cause noise during straight-ahead driving when the gears are unloaded. A worn pinion mate shaft can also cause a snapping or a knocking noise.

BEARING NOISE

The axle shaft, differential and pinion bearings can all produce noise when worn or damaged. Bearing noise can be either a whining, or a growling sound.

Pinion bearings have a constant-pitch noise. This noise changes only with vehicle speed. Pinion bearing noise will be higher pitched because it rotates at a faster rate. Drive the vehicle and load the differential. If bearing noise occurs, the rear pinion bearing is the source of the noise. If the bearing noise is heard during a coast, the front pinion bearing is the source.

Differential bearings usually produce a low pitch noise. Differential bearing noise is similar to pinion bearing noise. The pitch of differential bearing noise is also constant and varies only with vehicle speed.

Axle/Output shaft bearings produce noise and vibration when worn or damaged. The noise generally will be constant with bearings loaded or unloaded. Where axle/output bearing damage is slight, the noise is usually not noticeable at speeds above 30 m.p.h.

Intermediate shaft bearings produce noise when worn or damaged. The noise will changes when the axle is in disconnect. Road test the vehicle and actuate the disconnect. This will load the bearings and change the noise level. Where axle bearing damage is slight, the noise is usually not noticeable at speeds above 30 m.p.h.

 

[water4543]

GEAR NOISE

Axle gear noise can be caused by insufficient lubricant, incorrect backlash, tooth contact, worn/damaged gears or the carrier housing not having the proper offset and squareness.

Gear noise usually happens at a specific speed range. The noise can also occur during a specific type of driving condition. These conditions are acceleration, deceleration, coast, or constant load.

When road testing, first warm-up the axle fluid by driving the vehicle at least 5 miles and then accelerate the vehicle to the speed range where the noise is the greatest. Shift out-of-gear and coast through the peak-noise range. If the noise stops or changes greatly check for:

- Insufficient lubricant.

- Incorrect ring gear backlash.

- Gear damage.

Differential side gears and pinions can be checked by turning the vehicle. The side gears are loaded during turns. They usually do not cause noise during straight-ahead driving when the gears are unloaded. A worn pinion mate shaft can also cause a snapping or a knocking noise.

BEARING NOISE

The axle shaft, differential and pinion bearings can all produce noise when worn or damaged. Bearing noise can be either a whining, or a growling sound.

Pinion bearings have a constant-pitch noise. This noise changes only with vehicle speed. Pinion bearing noise will be higher pitched because it rotates at a faster rate. Drive the vehicle and load the differential. If bearing noise occurs, the rear pinion bearing is the source of the noise. If the bearing noise is heard during a coast, the front pinion bearing is the source.

Differential bearings usually produce a low pitch noise. Differential bearing noise is similar to pinion bearing noise. The pitch of differential bearing noise is also constant and varies only with vehicle speed.

Axle/Output shaft bearings produce noise and vibration when worn or damaged. The noise generally will be constant with bearings loaded or unloaded. Where axle/output bearing damage is slight, the noise is usually not noticeable at speeds above 30 m.p.h.

Intermediate shaft bearings produce noise when worn or damaged. The noise will changes when the axle is in disconnect. Road test the vehicle and actuate the disconnect. This will load the bearings and change the noise level. Where axle bearing damage is slight, the noise is usually not noticeable at speeds above 30 m.p.h.

Chrisbh17, thank you very much for this. Based on your descriptions, I think it's either differential bearings or axle shaft bearings since the noise is constant whether loaded/unloaded, and also does not fluctuate while disconnecting. I will do further testing and report back with my findings.