Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

1991 Toyota Pickup - Hums in 4X4

I have recently been driving in winter conditions in Alaska and have been utilizing my 4WD. I have hardly used the 4WD in the past because I haven’t had the need for it till now. However, when in 4WD and driving around 30+ mph there is a loud sound similar to a whining/humming noise when I let off the accelerator. The noise disappears when I accelerate again and returns as soon as I let off the accelerator. I have done a little reading but not much, could this have something to do with the front differential? Is it an issue that I should avoid driving with and go get it fixed?
Also, I have only driven a few miles like this but when I first put it in 4WD the noise occurred at any speed when I let off the accelerator, now it seems to only happen 30+ mph. Could this mean the 4WD just hasn’t been used in a long time so it just needs to work out the kinks? Thank you for any advice.

Is the tread wear depth even on all 4 tires?

The 4WD should be used only on surfaces that let each wheel slip a bit, such as on sand, grass, snow, gravel, etc. Never on clean hard pavement. This information is in your owners manual. Winter conditions often include long stretches of clean hard pavement and this causes a buildup of binding forces in the drive train, to its detriment.

1 Like

A year ago, my all terrain tires had uneven wear on the inside of all 4 tires. I had the alignment done and about 6 months ago I purchased new all terrain tires. After driving around 4,000 miles on them the tread seems to be wearing evenly.

Check the fluid level in the differential first, they can leak. If it’s old, change the fluid. If that doesn’t fix it, you might have other problems.

I understand the 4WD drive shouldn’t be used on dry pavement or surfaces that don’t allow the outside wheel to slip. The roads here have been super icy and I haven’t been able to accelerate out of a stop sign without 4WD. The last day or two the roads have cleared up and I am not using 4WD right now. So, with your comment about the “binding forces and the drive train, to its detriment” should I have a mechanic look at it or is it okay to keep driving with it the way it is until the noise gets worse?

Sweet, thank you for the advice. I will check the fluid and change it out. I don’t think it has ever been changed before.

It’s a good idea to get the right fresh fluid into the drive train components, and be very cautious in your use of 4WD.

I suggest you shift to 2WD and lift one front wheel off the pavement and then grab the driveshaft at the front U-joint and check for play by attempting to wobble the yoke on the input. If there is noticeable play one or both pinion bearings are worn and need immediate attention. Even when driving in 2WD a failed bearing in the differential can totally trash it and lock up both wheels.

Dang, okay I definitely don’t want that to happen! To check for play I only have to lift one front wheel not both?

If you are certain that the free wheel hubs are free neither wheel needs to be lifted but lifting one will ensure that the ring gear isn’t loaded against the pinion.

I ended up just taking my pickup into a mechanic to have a look. What he found when he checked the fluid in the transfer case and front differential was, the fluid was full but when he drained it there was metal shavings in both. He filled the fluid and sent it back home with me. His diagnosis was that either A) the transferase is bad and the part that is making the noise and needs to be replaced or rebuild. B) The front differential is bad and is the part making the noise and needs to be replaced and C) it could be just a front wheel bearing that is worn out and needs to be replaced. The interesting thing to me is since I drove away from the mechanic, the noise is actually worse. It isn’t any louder than it was before but it is noticeable at lower speeds now and has a little bit rougher of a hum. (Not totally sure how else to explain it)
The manual for the pickup says if the noise occurs when in 4H or 4L but not in 2WD it is most likely coming from the front differential (doesn’t say anything about the transferase). So given all of this information, should I start with replacing the front differential and go from their?

If the noise is there in 2 wheel drive with the hubs in the free wheel position the problem is not in the front differential and not likely in the transfer case. But I would guess that most mechanics familiar with 4 wheel drives could get the truck on a lift and quickly narrow down the location of the noise. I suggest finding an old mechanic with a lot of 4 wheel drive experience.

Humming/whining noises are often caused by the differential, front in this case, often the fluid is low, but it could be the bearings and/or pinion/ring gear too. The problem with the differential theory is that sort of noise is usually worse during acceleration and when you let your foot off the gas pedal it tends to diminish. So my guess is the transfer case. With metal shavings found in both the transfer case and differential, perhaps that’s the problem tjhat is causing weird symptoms. In which case - absent taking those things apart for a visual inspection of the gears and bearings — you might be able to get things working again to just flush all those metal shavings out. I doubt it though, you are probably looking at a rebuild on both the transfer case and the front differential. If you’ve driven in high water in the past, that could be the explanation. Water getting inside those will cause the gears to rust and become brittle. When I’d take my Ford truck through high steams on camping trips sometimes I’d have to replace the fluid in both differentials every night b/c of water intrusion. On mine the transfer case sits higher, so it seldom got water inside it.

Wheel bearing noises are usually described as “growling” which gets louder the faster you go. Tend to doubt that’s the problem here.

Thank you George_San_Jose1, Rod_Knox, and all others for the detailed posts, I am going to have another mechanic look at it and see what he says. I will relay this information to him and have him check out the front differential and the transfer case. I’ll let you know what he says and his advice.

This might be helpful. Also wheel bearing noises tend to get worse when turning one direction, so that’s something to test.