Another "What's that horrible noise" question...

[mwb9520]

Hi All - My first post and looking for input and opinions before putting the truck up on the lift this Wednesday.

1972 GMC Sierra, 1/2 ton, 2wd, LWB, recent crate motor and rebuilt Turbo 350, also new brakes front and rear.

No engine or trans noise at startup or idle. Nothing really noticeable driving at low speeds through the neighborhood at 0 to 35 MPH, but once accelerating up towards 40 I can begin to hear something like maybe a U-joint chirp, but noise doesn't increase much while under load all the way up to 50 MPH...but as soon as I let off the throttle it's like WTH IS THAT? Very loud grinding sound, but goes away again under load and at low speeds.

After reading some of the other posts on here, I was planning to R&R the U-joints and inspect the carrier bearing this week, but also wanted to know if there is possibly anything else I should look at while I'm under there...Thanks, Mick

[trac209]

I would say it's in the differential from what you say. Try accelerating and putting it in neutral while decelerating and see if unloading the diff makes the sound go away

[toolboxchev]

Your first instinct on U-joints is exactly what I would think also.

[davepl]

Gotta be between the trans output shaft and the tires, so perhaps U-joints, carrier, or less likely, the differential. But the good news is that diffs usually just whine... if it were grinding I think it'd fail pretty quickly and catastrophically!

My best guess though is the carrier bearing, they're a common failure point.

If your truck has a lot of miles on it, sure wouldn't hurt to throw all new U-joints and a carrier bearing at it. They're service items anyway, and then you'll know they're good.

If you get u-joints with grease fittings (zircs) be sure to put them in compression, but tension (so that when the truck engine tries to turn the driveshaft, it's squeezing one of them, not pulling it apart). Not a big deal but a minor thing that can help longevity under higher power applications, anyway...

PS: Nice looking truck!

[truckster]

I would check the u-joints, but that sounds a lot like what my differential was doing before the pinion blew through the case.

[demian5]

Originally Posted by trac209

I would say it's in the differential from what you say. Try accelerating and putting it in neutral while decelerating and see if unloading the diff makes the sound go away

Had the same thing. I'd guess this is it.

[HO455]

Welcome aboard. Good looking truck you have. Keep us posted on what you find.

[El Dorado Jim]

sub'd

[mwb9520]

Morning All - Appreciate all the input...Well, I had a chance to get it up in the air last night to have a look around. Found nothing obvious like slop or play, but the slip yoke behind the carrier isn't living up to it's name, so that could be a contributer. With the truck only having 72K on the odometer and having sat idle for several years while 'life happened', I'm guessing those 45 yo components are original and all due for replacement. I think I'll start cheap and easy with servicing the complete driveshaft assy, pull the diff cover and inspect it at the same time, and if that looks good, we'll see if that does the trick.

[mwb9520]

Just came back to close out this thread and say thanks to everyone that contributed.

In my case it was the U-joints...The worst being the one at the pinion yoke.

Although I did replace the carrier while I had the driveshaft assembly out, it was still in pretty nice condition...well, up until I was done with it. Also pulled the diff cover, inspected housing and ring/pinion, and all looked good. Fluid was old but not terrible, and no chunks of any kind. All clean and fresh now...and best of all...quiet.