hum or howl in 4wd

[DaveO9]

Vehicle: 2013 Tahoe LT. 207k.

I'm getting a hum or slight howl in both auto 4wd and 4-hi. Noise goes away (at least most) when letting off gas to coast or brake. Most pronounced when accelerating. noise stops immediately after switching to 2wd. I've only heard it in the driver's seat so far; I need to ask my son or wife to drive it and let me sit in the back and front pass. I think I'm hearing it up front. At first I thought it was the t-case chain and that it was normal. I still haven't fully ruled that out, but here's some more background about what I've done in the last year:

- replaced single speed t-case with 2-speed. Important: I had the same noise before replacing the t-case. So unless it's a normal thing with the magna t-cases, I don't think that's it.

- axle ratio swap, 3.08 to 3.42. Replaced both front and rear entire axle assemblies. Again, the noise was there before and after the swap. so I don't think it's the front diff.

So here's what's left that I think it could be. I haven't changed any of these components since I've owned the vehicle. (and I think the noise has been there since I've owned.)

- front driveshaft u-joints
- CV axles (do these make a hum or howl when going bad?)
- front wheel bearings/hub assembly (I see how they could hum, but it seems like they'd hum all the time, not just when under load?)

Thanks!

 

[rdezs]

What you describe.... The wine or howl disappearing when letting off the gas... Is a common symptom of a pinion bearing. And of course if it goes away when in two wheel drive, that would seem to point to the front differential pinion bearing.

Front hubs or rear axle bearings are typically pretty steady with speed, not affected by letting off the gas.

Was the front axle assembly you replaced rebuilt, or used?

I suppose another possibility is the transfer case front output shaft bearing also.... But I can't say I've heard one react the same way as a differential opinion bearing as far as letting off the gas. It's a roller bearing rather than the cone type found in the differential pinion bearing.

 

[Charlie207]

Your front differential bearings are bad. You could try draining, cleaning out, and refilling the front diff. fluid to get you a few more 4WD miles, but the sound won't ever go away or get better.

Luckily you can find junkyard units cheap. I found a good condition 3.42 (RPO code GU6, check your build sheet on the glovebox door) complete front diff on eBay for $205, and had free shipping from FL to NH. It's a relatively simple swap, just heavy.

One electrical connector, one u-joint, six axle bolts per side, and two nuts/two bolts securing the diff to the frame/subframe. I did the entire thing by myself and just pressed the diff up into position, and held it with one arm/shoulder while starting the nuts on one side, then repeating the process with the bolts on the other.

You could find someone to rebuild it, but the parts are probably going to be more expensive then an entire used unit.