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Help I.D the noise before my K5 selfdestructs

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lowlifeym

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I am getting a real loud ,(like people on the streets looking back) nasty noise out of my truck when I get off the gas to slowdown . It is a really loud growl/rubbing noise beyond gear whine noise . If I just barely touch the gas it pretty much goes away .
Truck is an 80 with a 350 , sm465 and np 205 . Front hubs unlocked , transfer case in 2wd , so I'm ruling out anything in front of the transfer case output shaft . It started fairly low key , like a tire whine . Front tires were done , badly scalloped , so 4 ball joints went in , left and right tie rod , alignment and 2 new tires . Front driveshaft has 3 new U joints and the rear 's are new also . It's gotten to the point where I'm afraid to drive it . I can feel it trough the bottom of the truck on the floor and through the steering wheel .
I have noticed that it is there all the time , even when at cruise speed , but gets really loud on deceleration and really bad decelerating in 1st and 2nd gear , alot less so from 3rd gear .
Transfer case is full of 80/90 and tranny is 1/2 full . Tailstock seal leaks into transfer case until tranny settles to 1/2 full . I got the truck like this 2 1/2 years ago and been meaning to fix it since I bought it . I filled the tranny to see if it made a difference and it makes none . I know , I should have fixed this earlier , but I am not 100% sure that the noise is coming from the tranny . When decelerating and I get the growl , I can push in the clutch and the growl does not go away . If it was tranny , the noise should go away ?!?!
Need suggestions here guys , thanks .
 
Only thing left to check is the rear axle and U-joints.

Pull the cover and check out the ring and pinon. Sounds like it may be your pinon bearing has failed.
 
Was it making the noise before the rear u-joints were replaced. If not, check them. I have seen u-joints installed dry. No lube from factory and not greased when installed. They wear out in no time.

If not, like said before, pull the rear cover.
 
First step is to pull the front shaft. That will eliminate any possible front driveline issues.

Sounds like the rear diff to me though.

bad mounts usually is the opposite. Youll get your weird noise or rub when on the gas not off.
 
I had a similar problem turned out to be the transfer case was not all the way in 2wd. It was in between 2wd and 4wd I pushed it all the way forward into 2wd and the noise went away. Try that:dunno:.
 
I noticed it more when pushing the brakes, slowing down.
 
had the same exact noise that you mention so i just pulled cover off the other day on my 85 k10 and found that the pinion was grinding the spool. time for a new rear.
 
Front driveline is pretty much eliminated , like I said , hubs are unlocked and transfer case is in 2 wd . Had someone look under as I pulled forward and the shaft is not turning , so I'm in 2 wd .
New rear U joints are heavy duty sealed units , so I can't grease them . Noise was there before , just not as bad .
I'm pretty good with the wrenches and have a well equiped tool box but you guys need to educate me a bit . Once I pull the diff cover to check the pinion bearing , what signs am I looking for ? Does the pinion have to be removed to inspect the inner race ?
 
Yes the pinion needs to be removed to check the pinion bearings. You should be able to get away with pulling the shafts out, dropping the rear shaft, and spinning the rear and feeling for resistence.

If somethings wrong your gear oil will be nice an metallic anyway though.
 
Easy way to check first, jack up the rear, grab the pinion yoke and check for play. Maybe even remove the driveshaft first for better feel. pull up and down and in/out. If it's flopping around, you got issues. Pull the rear cover and proceed from there.
 
It's possible your only problem is a loose pinion nut.
 
What is your pinion angle? Certain types of vibrations are much worse under near zero load. And sometimes if are under engine braking or braking your pinion angle can get even worse because it wants to rotate the other way.
 
It keeps making the noise if I push in the clutch , but it doesn't get any worse . Pinion angle is "stock" , truck has no lift and I have 31's on it .
 
Front wheel bearings. I had a grinding noise on decelleration that I could feel in the floor, and would all but go away while driving. Turned out to be a spun wheel bearing up front. I guess the weight transfer under throttle was enough to make the sound go away.

Just another idea :waytogo:
 
Alright , I think I've found the problem . Grabbed the pinion yoke , gave it a shake and it has lots of play and my U joint caps have play in the pinion straps !? The U joint is less than 2 months old , why there is play within the strap is a total mystery .
I pulled the diff cover and staring back at me is the most beautiful shiny pinion gear wear pattern on the entire outside of the carrier that you ever did see ! It is worn in there pretty deep .

1st question ,
is this mess salvagable or just look for a used unit from the wreckers ? Gears don't seem to have chips out of them , couple good nicks on the outside edge of the ring gear where the gear ratio is stamped , but the pinion's teeth on the edge seem worn a bit on the edges , kind of if I stood it on it's end on a table , where the edge of the gears are would not be 100% flat to the table .

2nd question ,
on the edge of the ring gear here are the numbers I found . 459798 ? that number is questionable because at least 2 of the numbers are a little messed up . I think these are part numbers anyway because the other numbers I found are 17 47 GM 10 94 .
This confused me because I assumed it should have been 47 17 . Dividing 47 by 17 gives me a gear ratio of 2.76 !? Didn't beleive that so I counted teeth . Sure enough , 47 ring , 17 pinion for a ratio of 2.76 ! I thought blazers came with 3.08 3.73 or 4.10 ? I didn't even know GM made a 2.76 , I thought they were a 2.73 ?
 
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