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1994 Chevy K1500 4.3L, 340K KM
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Discussion Starter · #1 ·
I recently purchased a 94 K1500. 210K miles. The truck was sitting for a few years and fixed up by a local mechanic.
The truck runs well, for it's age, might be burning up a bit of coolant.

One worry is the "hum" from the transmission. What I find interesting is this: When coasting in gear the truck is quiet. however as soon as you touch the accelerator the hum appears. Release the accelerator and its gone.

I'm concerned I'm losing a bearing or universal joint. Anyone have a guess where to start to look?
 

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2001 Suburban 5.3L, 302.2k miles
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The hum may not be from the transmission but rather from the rear differential if in 2WD mode. If in 4WD mode, the front differential comes into the scene but its rare unless it ran low on gear oil or never been replaced in its previous ownership....

The hum can also be from the torque converter.

I check all fluids when I buy used vehicles.
 

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1994 Chevy K1500 4.3L, 340K KM
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Discussion Starter · #3 ·
Thanks,
The guy who restored the truck said he did front and rear differentials. I might go ahead do them again anyway. I changed the transfer case and transmissions, while both were fairly old they weren't low.

The noise is there in 2wd and 4wd, if that matters.
 

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At that mileage, the differential bearings are probably worn.
I changed my rear differential bearings at around 200k miles I think.
 

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2000 Silverado Z71 4x4 5.3L 480k+ miles w/ GM rebuilt motor and trans
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When coasting in gear the truck is quiet. however as soon as you touch the accelerator the hum appears. Release the accelerator and its gone.
A U-joint would present this way. You're unloading the U-joints by coasting and letting the transmission "fall behind" the rear differential, unloading the U-joints. Once you touch the accelerator, the transmission "catches up" to the rear differential and begins applying force to the tires again through the U-joints.

There are also things inside the rear differential which could be loaded/unloaded in the same manner, but I've only ever seen this symptom manifest itself in my truck due to the U-joints.

Side note: If you have manually locking hubs, then your front driveshaft is probably not the source of the issue. In my truck, the hubs do not unlock and the front driveshaft spins at all times, but it's disconnected from power inside the transfer case. My front U-joints often fail at the same time as my rear U-joints due to their constant turning, though I'm rarely in 4WD.
 

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1994 Chevy K1500 4.3L, 340K KM
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Discussion Starter · #6 ·
A U-joint would present this way. You're unloading the U-joints by coasting and letting the transmission "fall behind" the rear differential, unloading the U-joints. Once you touch the accelerator, the transmission "catches up" to the rear differential and begins applying force to the tires again through the U-joints.

There are also things inside the rear differential which could be loaded/unloaded in the same manner, but I've only ever seen this symptom manifest itself in my truck due to the U-joints.

Side note: If you have manually locking hubs, then your front driveshaft is probably not the source of the issue. In my truck, the hubs do not unlock and the front driveshaft spins at all times, but it's disconnected from power inside the transfer case. My front U-joints often fail at the same time as my rear U-joints due to their constant turning, though I'm rarely in 4WD.
That's really interesting about the U-joints. The truck doesn't have locking hubs - just 4wd on the floor. I had read about the symptoms and most sites said it would be a 'clunk' sound. As I sound, shes quiet as a mouse when coasting, and then a loud 'hum' as soon as the you touch the accelerator.

I'll get underneath and check the U-joints for play. I'm sure they are ancient.
 

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1994 Chevy K1500 4.3L, 340K KM
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Discussion Starter · #7 ·
At that mileage, the differential bearings are probably worn.
I changed my rear differential bearings at around 200k miles I think.
I agree those will need to be done. Would that 'hum' be present all the time, or only when the gas peddle is touched, if it were the bearings?
 

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2000 Silverado Z71 4x4 5.3L 480k+ miles w/ GM rebuilt motor and trans
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I had read about the symptoms and most sites said it would be a 'clunk' sound.
It could go either way. I've had both types of failures.

The clunk-clunk-clunk failure was a burnt up, crushed needle bearing. This was present at lower speeds and eventually became a disturbing crunching at high shaft RPM's.

The failure that sounds like a failing wheel bearing variable pitch hum was just a dry, worn out set of needle bearings.
 

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1994 Chevy K1500 4.3L, 340K KM
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Discussion Starter · #9 ·
It could go either way. I've had both types of failures.

The clunk-clunk-clunk failure was a burnt up, crushed needle bearing. This was present at lower speeds and eventually became a disturbing crunching at high shaft RPM's.

The failure that sounds like a failing wheel bearing variable pitch hum was just a dry, worn out set of needle bearings.
thanks.

this is very much a wheel bearing kind of noise (but clearly not a bearing).

I’ve done a couple u-joints before and it shouldn’t be too tricky hopefully. Certainly a lot less money than replacing gears!
 

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1994 Chevy K1500 4.3L, 340K KM
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Discussion Starter · #10 ·
Did a little more checking, I can’t detect a loose u joint- but I suspect that doesn’t mean much. Might still do them anyway. Hard to tell what to buy however- rock auto has a ton of different options.

there is some play in the front axle coming out of the transfer case. There is a join there after the u joint and it’s a bit sloppy (also has a grease nipple on it). Not sure if it’s supposed to be that way (I’m used to working on two wheel drive cars.

driving The truck today I did notice a few things. I was comparing noises in 4wd vs 2nd. (This truck is a “4 on the floor”).
In 2nd the whine/hum is present about 25-30mph, and goes away as soon as you release the gas peddle.
In 4wd, it’s the same, but perhaps a little less noticeable. However-in4wdthere is a definite “grind” when braking. I though this was just old brakes- but it isn’t there in 2nd.
 

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Did a little more checking, I can’t detect a loose u joint- but I suspect that doesn’t mean much. Might still do them anyway. Hard to tell what to buy however- rock auto has a ton of different options.

there is some play in the front axle coming out of the transfer case. There is a join there after the u joint and it’s a bit sloppy (also has a grease nipple on it). Not sure if it’s supposed to be that way (I’m used to working on two wheel drive cars.

driving The truck today I did notice a few things. I was comparing noises in 4wd vs 2nd. (This truck is a “4 on the floor”).
In 2nd the whine/hum is present about 25-30mph, and goes away as soon as you release the gas peddle.
In 4wd, it’s the same, but perhaps a little less noticeable. However-in4wdthere is a definite “grind” when braking. I though this was just old brakes- but it isn’t there in 2nd.
I would have to say that you have a front wheel bearing going out. The grinding noise that your hearing when you break is the hub assembly shifting and binding up on the bearing.
 

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1994 Chevy K1500 4.3L, 340K KM
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Discussion Starter · #12 ·
Just to share, it turned out the rear end noise was the rear dif.
The crush sleeve was completed gone. After some new bearings, new crush sleeve, and a new set of spider gears everything is running smoothly
 

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2001 Suburban 5.3L, 302.2k miles
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Just to share, it turned out the rear end noise was the rear dif.
The crush sleeve was completed gone. After some new bearings, new crush sleeve, and a new set of spider gears everything is running smoothly
Good job!

Reminded me of my 2001 suburban with pitted differential pinion bearings, it was loud I stopped driving it and immediately worked on the differential before the gears exploded.
It's now at 302k miles and still runs like new on stock engine and 4L60e. Around town I only get 15mpg, but it's paid off. It just needs gas and oil changes, well fuel pump maybe soon.
 
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