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1981 GMC Caballero 305/TH350
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Discussion Starter · #1 ·
Hey Everyone,

I have a 1981 GMC caballero with 305/th350 combo. Typically when I drive the car over 40mph it will begin to make a humming noise, that loudens when I put my foot on the gas. The sound sticks around until around 60mph then it goes away.
I have a 10 bolt rear and im not sure what kind of guts it has in it.
As of recent the humming has got worse, it has begun to hum as low as 30mph.
Any ideas? All I can guess is its about time for some new wheel bearings and a diff fluid change. Just hope it has nothing to do with my transmission.
 

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Hey Everyone,

I have a 1981 GMC caballero with 305/th350 combo. Typically when I drive the car over 40mph it will begin to make a humming noise, that loudens when I put my foot on the gas. The sound sticks around until around 60mph then it goes away.
I have a 10 bolt rear and im not sure what kind of guts it has in it.
As of recent the humming has got worse, it has begun to hum as low as 30mph.
Any ideas? All I can guess is its about time for some new wheel bearings and a diff fluid change. Just hope it has nothing to do with my transmission.
Check your engine cooling fan/clutch assembly.
 

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Deputy Regional Director, Region 13
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6,622 Posts
humming noise ? and it only happens between 40-60mph? what engine rpm is it at during those speeds? does it hum at the same engine rpm at a lower speed? have you tried comparing 3rd and 2nd gear at those speeds?
 

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1981 GMC Caballero 305/TH350
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Discussion Starter · #6 ·
humming noise ? and it only happens between 40-60mph? what engine rpm is it at during those speeds? does it hum at the same engine rpm at a lower speed? have you tried comparing 3rd and 2nd gear at those speeds?
My dash does not have a tach, so I wouldn’t know what rpm. Only 40-60 will it hum. Doesn’t matter what rpm. I will give that 2nd gear idea a try soon though.
 

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No Idea. I would guess factory stock. Would bad U joints cause a clunking noise when first shifting into reverse?
Yes, but to much play in the rear would do the same thing.Chock the wheels and put it in neutral and turn the driveshaft by hand back and forth to see if the play is in the rear or driveshaft. If you do it close to the rear you should be able to hear the play. Driveshaft should not wiggle.
Tom
 

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Deputy Regional Director, Region 13
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My dash does not have a tach, so I wouldn’t know what rpm. Only 40-60 will it hum. Doesn’t matter what rpm. I will give that 2nd gear idea a try soon though.
the reason for trying a different gear at the same speed is to determine if it is engine speed or transmission speed related. if the noise is still there, you know what you can rule out. and while it may be the things others have suggested, it could also be a noise-induced harmonic by resonance of wheels or body.
I had a similar issue years ago where the noise would only happen above 50mph. it was finally tracked down to the sideview mirror mounted on the drivers front window corner. apparently there was a slight deformation of the plastic mount during manufacturing, and at speeds above 50mph the air would finally force an entry along the edge causing it to wedge open and act like a low-pitched whistle.
 
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