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Just starting to play with my m37 at it is very loud when under power. Not when coasting, not when revving engine while out of gear. This leads me to think it is transmission or transfer case. It works fine and seems smooth, but it is a loud hum, bordering on a roar. I don't have anything to compare the sound to, so I don't know if it is normal.
Are these always loud?

I haven't checked fluids yet, that will be next as soon as I figure out how.
-wwiifirearms

they are pretty noisy rigs but if that bad could be rear diff. when rear diff gets worn out, bad teeth, beaings etc it sounds like tranny or tcase becausethe noise travels up driveline. I was convinced once that my tcase was bad but it was the diff..... and 5.83 gears live a hard life. it takes double the abuse of a half ton with 3.08s. if rear diff gets run dry it fries the teeth and they get real noisy. many times the bearings are in good shape though still. huge bearings and 3 on the pinion. If rembering right you cna check the ring gear teeth throught he filll hole. should be nice and smooth look for wave pattern in the teeth. if wavvy will sound like a machine gun.

Transmission fluid was filled at least to the upper rear cap screw on the right side PTO cover.
T-case was about a quart low. I had SAE 30 around, so I topped off with that for now.
Haven't checked difs yet. It got dark and lights aren't working so I'll have to test drive tomorrow.

Side note: do people generally fill the transmission to the PTO cap screw, per the manual, or fill all the way to the fill hole?

-wwiifirearms

Update: some more test runs today and I found the truck is much quieter with the front axle ENGAGED(one lever back and the other forward, should be high range 4x4 right?)
This makes me think there is some t-case issue.
Any suggestions?
-wwiifirearms

that would certainly see like a T-case issue. Probably time to pull it apart

Any suggestions on a shop to rebuild it near Cleveland Ohio. I could get it out/in, but I don't have the time or special tools for the rebuild. If reasonable I'd have a pro do it.
-wwiifirearms

still could be rear diff. when you engage front axle you take half the load or more off the rear. easy to check teeth through fill hole.

If the t/case was a quart low on oil, you can expect that is high on the list for likely problems. The case only holds 2.5 quarts when full, a quart low is huge, typically damage is done to the 2-speed side of the case when oil is low. That side of the case is higher and when oil is low, that side goes lacking. Doesn't take much running like that to cause some real problems.

We offer rebuild service for all drivetrain components. Let us know if we can help; comes back to you with a 12 month warranty.

Thanks to everyone for the info. I'll have a look at the diff gears before I get too far into things.
MSeriesrebuild, Sent you a PM.
-wwiifirearms