Is my transmission going kaput?

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LVJJJ

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I had a hum in my '94 Suburban k1500, I swore it was coming from the front, so replaced hubs, still had it. Turns out it was in the differential, needed new small gears (not the ring & pinon) and seals. Solved it. Driveline shop told me to keep the fluid fresh.
 

Calamus

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Lots of great replies here:

@SUBURBAN5 : I personally did not do the seals, they were done by a friend of mine. He said he checked the bearings, but no, we did not check the pinion other than a cursory visual inspection. This makes me very curious...

@Nad_Yvalhosert : What you're describing is exactly what I'm hearing. Sound is loudest at 50-70kph, and I hear it with steady throttle, or soft pedal increase, and it disappears entirely when i let off the gas to slow down slightly. It sounds like I have to seriously inspect that rear differential. We did replace the seals, and when we did we noticed that the previous owner had really farm fixed the rear diff. In fact, he had sheared 2 bolts on the diff housing, that were still stuck inside the bolt holes. We had to get new bolts, and it was a pain in the figurative butt to get those old sheared bolts out... Problems in the diff would not surprise me.

@Pinger :I don't hear any difference in noise when applying the brake, regardless of if I apply without any throttle, or if i slightly apply while maintaining throttle. When you say the axle is a possibility, are you suggesting the axle itself? I'd mount my action cam up under there and take it for a drive, and upload the vid for you to listen to, but there's rather a snow storm that blew through here last night, so I have to wait for weather to smarten up, lol! If it were the axle, am I risking any sort of catastrophic failure to drive it around?
 

SUBURBAN5

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Lots of great replies here:

@SUBURBAN5 : I personally did not do the seals, they were done by a friend of mine. He said he checked the bearings, but no, we did not check the pinion other than a cursory visual inspection. This makes me very curious...

@Nad_Yvalhosert : What you're describing is exactly what I'm hearing. Sound is loudest at 50-70kph, and I hear it with steady throttle, or soft pedal increase, and it disappears entirely when i let off the gas to slow down slightly. It sounds like I have to seriously inspect that rear differential. We did replace the seals, and when we did we noticed that the previous owner had really farm fixed the rear diff. In fact, he had sheared 2 bolts on the diff housing, that were still stuck inside the bolt holes. We had to get new bolts, and it was a pain in the figurative butt to get those old sheared bolts out... Problems in the diff would not surprise me.

@Pinger :I don't hear any difference in noise when applying the brake, regardless of if I apply without any throttle, or if i slightly apply while maintaining throttle. When you say the axle is a possibility, are you suggesting the axle itself? I'd mount my action cam up under there and take it for a drive, and upload the vid for you to listen to, but there's rather a snow storm that blew through here last night, so I have to wait for weather to smarten up, lol! If it were the axle, am I risking any sort of catastrophic failure to drive it around?

Biggest thing is to narrow it down. If you have a camera you could always attach it underneath and drive it and see if it picks it up
 

Pinger

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@Pinger :I don't hear any difference in noise when applying the brake, regardless of if I apply without any throttle, or if i slightly apply while maintaining throttle. When you say the axle is a possibility, are you suggesting the axle itself? I'd mount my action cam up under there and take it for a drive, and upload the vid for you to listen to, but there's rather a snow storm that blew through here last night, so I have to wait for weather to smarten up, lol! If it were the axle, am I risking any sort of catastrophic failure to drive it around?

I think, the reason a wheel bearing can change sound with the brakes applied is due to the bearing loading shifting to a different place where there is less wear. Which is why I suspected them with the change in noise on or off the gas.

My experience of failing wheel bearings is noisy over a speed range (quietening above a certain speed) but very hard to detect at the wheel - roughness before play if anything detectable. Then they will make a horrendous noise (like a fighter jet flying overhead) during driving and go quiet again. At that point what has happened can be detected at the wheel and the bearing has to be changed.

It's a bearing - and there's bearings in the diff also.... But there's also a suspicion of pinion wear and that ties in more with the change in noise on or off the gas.

Diffs have been known to seize during driving and lock the rear wheels so some caution required here. I'd try - after a run when the diff is hot - to extract some oil via the level plug from the bottom of the oil and check for metallic particles. You'll need some suction device for this. Swarf would indicate a problem - lack of doesn't necessarily say there's nothing amiss but it's a case of zoning in on any available info.

Might be worth checking the input flange for play/roughness there that could suggest bearings there giving up the ghost.
 

Calamus

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Sorry for not updating sooner. About 3 weeks ago now I replaced the brake switch. It fixed my cruise control issue, so yay for that. Hasn't affected my sound when driving though, so I guess that narrows it down to the pinion bearing being the most likely.

Seeing as it's -35 C (something like -41 F or so) today, and the last time I went for a drive in this weather the sound nearly doubled, it lends credibility to the pinion bearing, I think, although it also means I don't want to drive it in this weather. She'll sit for now, probably all of Feb, and once the weather starts to warm up regularly I'll give a shot at that pinion bearing. Will update then.

Thanks everyone who's given their opinions in this.
 
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