84 k10 shotbox with lift. trouble with a noise. Drive line angle? Transfer case?

[84Silverado4x4]

Hey guys, My 84 chevy k10 shortbox is giving me a noise problem and I am trying to run it down.  I don't know the exact amount of lift but it is all suspension between 6-8".  I have a 5.3 ls swap with 4l60e trans and np 208 transfer case.  I have the t case shimmed best I can at 5 degrees down.  The rear 12 bolt axle is at 11 degrees up.  I tried a 6 degree shim to try and bring the rear to match the transfer case but the u joint bound on the pinion nut.  My noise comes when I am in a coast state and I typically only hear it in OD coasting at 40+ mph speeds.  I can pull it down into drive and the noise goes away.  I can rule out the transmission, and rear end as both have been replaced since I have owned the truck and the noise has been there ever since I have owned it.  I have also had the t case rebuilt but the noise didn't change.  New drive shafts.  I have also had the truck to two different shops and they did different tests and both swore it was the rear end but I fully rebuilt it with all new parts minus the housing and the noise is still there.

Does anyone have any ideas for my next approach to fix this noise?

[dvdswan]

what kind of noise is it?  howl, whine, growl, etc. 

[fitz]

If you think it's the rest end I would find a 10 bolt to install temporary and see if the noise goes away.
Any gear ratio will work, just don't use  4 WD.
I would think you can find a 10 bolt dirt cheap at the junk yard or on craigslist.  If you were in Massachusetts  you could come by and I'd give you one from a parts truck.  It's a cheap way to see if the repair shops are correct and your rear end is bad.  Just because it's been rebuilt doesn't mean that it will be perfect.  There is allot to getting the gears set up correctly.

[AZ4X4SQBDY]

I would place the vehicle on 4 sturdy jack stands under each corner of the axles, get the tires off the ground, have somebody run it up to speed and create the problem. I would then use 3 or 4 feet of heater hose, hold one end to my ear and place the other end near the various bearings in the driveline to isolate the noise.

Doing this can be as safe or as dangerous as you choose with the big tires and drive shafts spinning at 40-50 mph. I would use enough hose length so that I would not have to be under the truck, I would be off to the side so if something did happen, it would just drive away in front of me, not over me. The length of hose would have a stick or rod taped to it to support it reaching the differential, trans and transfer case. I just don't know of another way to check the driveline running with weight on the axles.

[84Silverado4x4]

Both shops did some different 2 wheel and 4wheel tests.  They both seemed to think it was coming from the rear.  I had the one shop dig into the rear end to see what we could find and if they could fix it.  They found that the GM 4.56 gears were not set properly and for the first attempt they just tried getting the gears set right.  That didn't make a difference and they said the gears must be warn in that way and I needed new ones.  I then bought a new set on Motive 4.56 gears and all new bearings for them to try.  After installing them it made no difference.  The truck has a earlier model 12 bolt in it and it had a detroit locker.  They thought it may have been possible the locker was chattering.  They told me we should try a new different style locker next.  I purchased a Detroit Trutrac locker and they installed that.  That made zero difference.  So aside from the rear end housing and axles every item in the rear end has been replaced.  The truck came with a 350 and 700r4 tans that I swapped out for the 5.3 vortec and 4l60e trans.  The noise was there before and still remains after the swap.  I also had the NP208 t case rebuilt.  I know they replaced the chain and guides.  The truck also had a pinion angle shim of about 6 degrees in the rear pitching it upward.  I removed that and gave it a try and no change.  I also tried adding a 6 degree shim to it to correct it the other way and match the angle of the transfer case but the angle is too steep and the rear ujoint was binding on the pinion nut on the differential.

The noise I get is hard to describe but it's almost like a power steering grown.  I used the example it sounds like an old fishing rod letting out line out when you have a bite.  The higher the speed the louder the noise but it's only when I am in a coasting state.  Mostly does it in OD until the truck down shifts.  I can manually shift the truck down into drive and it instantly goes away.  It will also do it in drive but only for a split second when going from accelerate/maintain speed to letting off the throttle and coasting.  This noise sounds like it's right underneath the cab and it has a slite vibration from the harmonics you can feel in the floor.

Can my driveline angles be off causing this kind of noise?  is there something in the T case that could make this noise?

I need to be driving about 40 mph + to get it to make the noise.  And I can go as fast as 100 mph and it will not make the noise until I take my foot off the throttle, but the second I do the noise is there.

[zieg85]

Have you ruled out exhaust rpm related like a engine brake type sound?

[84Silverado4x4]

Yeah this is definitely not exhaust.  The truck will have a little exhaust popping on decel when I pull it down into drive from OD to stop the noise which I do on a regular basis because the noise is getting so annoying. l

[roundhouse]

A whine when coasting usually means the gears are not set up quite right

Too much backlash or pinion depth or something

Setting up diff gears is the brain surgery of the automotive world

You gots to be an expert with dozens of them as experience
To get it right

That said
If you have 8" lift I'm assuming you have 35-36-37 tires ?

A ten bolt is ok in a 6 cylinder chevelle but they don't last long in pickups even with stock 28" tires
The back axle is the first thing that gets sold out of a square body at a junkyard

Too bad a nine inch ford axle isn't a bolt in
Those are very strong

If it keeps giving you problems you can find a 14 bolt from some certain models of Chevy Avalanche that had 6 lug wheels





Sent from my iPhone using Tapatalk

[84Silverado4x4]

It's a 12 bolt rear axle.  It isn't gear whine I know that and the shop knows what they are doing.  I have had gear whine in some other vehicles. 

Tires are 35x12.50x18's Toyo MT's

lift is somewhere between 6-8"s and is all spring no blocks.  I have a hunch they are rough country springs but I do not know that for sure.  I had another 84 shortbox with a 6" superlift and this one sat a touch taller for some reason with the same size tires.

[roundhouse]

It's a 12 bolt rear axle.  It isn't gear whine I know that and the shop knows what they are doing.  I have had gear whine in some other vehicles. 

Tires are 35x12.50x18's Toyo MT's

lift is somewhere between 6-8"s and is all spring no blocks.  I have a hunch they are rough country springs but I do not know that for sure.  I had another 84 shortbox with a 6" superlift and this one sat a touch taller for some reason with the same size tires.

12 bolt is not any stronger than a 10 bolt

The noise Might be from the t case

Also check the axle bearings out at the brake drums


Sent from my iPhone using Tapatalk

[84Silverado4x4]

I thought a 12 bolt was always considered stronger than a 10?

They installed new axle bearings when they had the rear end apart.

When I had the T case rebuilt they said one of the bearings had a slight rough spot in it from sitting a while without use but they felt they got it working freely and didn't need to be replaced.

What would be making the noise in the T case?  It has the noise in both 2 and 4 wheel but the tone of it does change ever so slightly.