Transfer case noise in 88 suburban.

[Rockhounder]

Hey there guys.

I have a shushing noise in my transfer case on my 88 K1500 suburban (350 motor, 700R4 tranny)

It is most noticeable around 35 to 45 miles an hour It almost sounds like it is same frequency as wheel rotation cycle, so at first I was thinking that it was probably coming from differential, and when we put it on the rack, the noise was gone, even when letting the wheels spin at 40 mph. Then we had a friend hang out the rear door, on an empty stretch of road at 40 mph, and he definitely said the noise was coming from transfer case area.

When it is on the rack however, we can hear only a very very slight loose bearing type of noise coming from rear side of transfer case. When it is on the road, with windows down, it is loud enough that it can be heard very easily, especially when we drive up against the center concrete barriers, or up against another vehicle. It has been getting slightly louder over the last month or so. When I put it into 4wd, and drive straight on the road, the noise gets 75% quieter. We have recently replaced both rear driveshaft U joints. The noise sounds almost like a metallic "shushing" sound, about 4 times a second while driving 40 mph.

Anybody have this problem ,and figure out the cause? I was thinking of going ahead and replacing transfer case bearings. Any advice is appreciated!
Thanks!

 

[1stDeuce]

Locking hubs or "auto" hubs? If locking, does the noise go away if you unlock them? Has the t-case ever been run really low on fluid? If either of those are a yes, it may be the chain slapping the case. That's bad, and will lead to needing a complete new case if you don't get a new chain installed ASAP, or keep it out of 4wd, and keep the hubs unlocked.

If auto hubs, the chain shouldn't be spinning much when you drive unless the hubs are messed up... Pull the fill plug. Stick your little finger or a piece of wire in there and see if the chain is really loose. While you're at it, see if there's any fluid left in the the t-case, or do a drain and fill and see if the fluid is metallic looking...

Shake the front and rear outputs to make sure the bearings are tight. Check the U-joints too. When they first go bad, they'll "squeak" twice per revolution... Then they get quiet until they fail totally. If the driveshaft has greasable u-joints, grease them!

Those are my ideas for now.

 

[Rockhounder]

Locking hubs or "auto" hubs? If locking, does the noise go away if you unlock them? Has the t-case ever been run really low on fluid? If either of those are a yes, it may be the chain slapping the case. That's bad, and will lead to needing a complete new case if you don't get a new chain installed ASAP, or keep it out of 4wd, and keep the hubs unlocked.

If auto hubs, the chain shouldn't be spinning much when you drive unless the hubs are messed up... Pull the fill plug. Stick your little finger or a piece of wire in there and see if the chain is really loose. While you're at it, see if there's any fluid left in the the t-case, or do a drain and fill and see if the fluid is metallic looking...

Shake the front and rear outputs to make sure the bearings are tight. Check the U-joints too. When they first go bad, they'll "squeak" twice per revolution... Then they get quiet until they fail totally. If the driveshaft has greasable u-joints, grease them!

Those are my ideas for now.

It's auto locking hubs, and the T-Case was just recently fluid checked, ok level, but not checked for metallic color. I will do that. I am leaning towards going ahead and getting an all moving parts rebuild kit. The U Joints we put in about 600 miles ago are the new, grease nipple type, and fully greased up. When we took out the old U joints, the needle bearings had worn literally half circle grooves into the U joint centers, looked like a double crosshatch groove pattern. We had been pulling heavy loads up to big bear before the noise (moving up to big bear) and I think those heavy lads on the drivetrain might have caused the bearings on the T-Case to wear and loosen up until there is slap?

 

[1stDeuce]

Strange that bearings would make a repetitive sound. The chain shouldn't slap in 2wd... could it be brakes or front or rear wheel bearings? Rear more likely if its a half ton...

 

[cyclic]

Lifted?

Also, try pulling the front drive shaft and go for a drive.

 

[rayra]

chain loose, or link pin shifted, rubbing on the inside of the case?

 

[hidesertwheelin]

If it becomes a big problem, I have a NP241 that I just rebuilt a few years ago (except for the chain). I only ran it a few hundred miles before I scrapped the rusty truck. All it would need is a speedo gear (it's out of a 1990, so it's has the electronic speedo fitting). I live down the back side of the mountain in Hesperia.