Need help diagnosing something. I've just put in a new-to-me transfer case. I was told it was professionally rebuilt with very little mileage before it was pulled from another EB. The gears looked better than my old case, turned smoothly by hand and it isn't particularly dirty, so I'm fairly sure this is true. I'm getting a whine from the transmission tunnel that I thought I had before but is now much more pronounced. It turns up around 45 or 50 mph and is only present under light load, steady state driving (think constant speed on relatively flat roads). Haven't had the truck up to freeway speeds since I did the swap, so not sure if it gets worse as I go faster. Not super loud, but it is audible. I'm thinking there are 4 possibilities:

1. I'm nuts and hearing things. Not entirely impossible. I thought I was hearing this noise out of my old case, but it was quiet enough that I'm not sure. Phil couldn't hear it when I took him for a ride, but had previously confirmed that the old case needed replacing based on the bearing wear when we re-shimmed the rear output shaft.
2. The new case is just tighter than the old case and this is normal or something needs to break in.
3. I've got a bad bearing/gear somewhere and the new case has problems.
4. Something else that someone smarter than me will think of.

I'd like to drive it to work tomorrow, but am not interested in dealing with a broken down truck during work hours and would appreciate any input.

Did you try running it in 4x4 to see if it changes?

Also try disconnect the rear shaft in 4x4 and see if noise goes away.

I doubt your hearing things, if hear a noise unfamiliar there is something wrong.

The case initially made a little noise around 25 or 30mph that I figured was just oil still working its way into the bearings after sitting for a long time. Went away after a mile or two. I put it in 4wd then without much change. I'll try again and see if it is still present.I've probably put a total of 15 miles on the case since install, not sure if this will work itself out with time and mileage. I don't want to toast an otherwise good set of gears.

Justin, what type of fluid are you using in it? ATF will make them suckers noisey. I have a long story about this from my 78.

80-90w gear oil.

Could be the bearing in the adapter. Did you swap that?

Yea, between the constant ringing in my ears and the roar of your tires I couldn't hear it.

No I didn't, was thinking that might be the problem. Any idea why swapping the t-case would make it worse? Just more load due to the tigher (or existing) tolerances? Given the shape the bearings in the old t-case were in I probably should have.

If you didn't swap the bearing you probably didn't swap the input gear either. You might be getting some extra noise there because you have 2 gears with slightly different wear patterns meshing together now.

Nope, didn't. Didn't know that was important. Can I get away with just letting them wear into each other, or will that mess something up?

I know people do that all the time and I haven't heard of any failures there. I like the gears married to the tcase though.

I don't have the input gear that goes with this t-case. Would have installed it and changed the bearing if I did. Any other option than to let the problem work itself out by letting the gears wear in?

Is that part of the fusion transfunctioner?

Finally got some time to get the truck out and run it some more. The whine is only present from about 45-55 mph under light throttle. I suspect it goes away above 55mph because air resistance creates enough load on the drivetrain to get rid of it. Its quiet, but is definitely there. I'm wondering if I'm hearing normal gear noise that didn't happen with my old case because the bearings were fairly worn. Is that a reasonable assumption? May need to bring it down to a Bronco-smart person's house for a ride to see if I'm crazy. The truck otherwise seems to be driving normally.

I'm sure this is not the problem, But I have in my original manual that a manual tranny runs the 80 90w but the transfercase should be using a 50w motor oil? that correct because thats what I'm running in my 66' By the way Klotz 75w-140 Quieted down my rear end and tranny, at hyw speeds with 4.11 gears and a possi I would start getting a friction whine even with the friction additive in it.

I appreciate the thought, but I'm pretty sure that isn't it. Ford switched to 80-90w sometime in the mid 70's. My understanding is that either is fine. I guess the 50w is better for mileage and the gear oil provides a little more protection.

I'm an idiot. May have discovered the source of my problem. One of my hubs was locked in. I'll have to see if it is still making the noise once I get the front end back together. Let the abuse begin....

That is nice you found a possible solution. Not going to get any abuse out of me - I am sure I would have just turned the stereo up so I couldnt hear the whine.

Maybe that's my problem-I don't have a stereo.

Bringing this thread back from the dead. I'm not insane (wasn't sure there for awhile) and the whining noise I'm dealing with is getting worse. It shows up mostly in 3rd gear between 40 and 50 mph. It may be there at higher speeds and I just can't hear it over my tires. I'm thinking it may be the bearing in the adapter, but am not sure why swapping the t-case would cause this to happen. The only other thing I can think of is that I got taken and the t-case I've got wasn't ever actually rebuilt and has a bad bearing. Any ideas?

Sounds more like wheel bearings or carrier bearings in the rear end.

Many thanks to 777mechanic and Viperwolf. They helped me out big time by taking a listen to it, looks like Zoso hit it on the head with something being out of whack in the rear end. Looks like the next investment will be a rebuild kit for the 3rd member and some wheel bearings and seals.