2001 GMC W3500 Forward Loud Squealing/Squeaking

Hello everyone.

I have a 2001 GMC W3500 box truck that I got at a real bargain. Replaced the battery, air filter, plugs, wires, distributor with cap, and ignition coil. It would not start. It was in need of an engine overhaul (didn't have enough compression to start). I had the engine replaced with a used one.
Recently it turned off on the highway. After I let it sit for 1.5-2 hours, it still wouldn't start, and it made like a bad starter squeaking noise (not like a drill, but like a squeak), so I had to have it towed . I replaced the radiator and thermostat although it didn't mark hot on the gage, just had previous plans to do so, and also the starter. Now, it starts and runs when cold and sounds like it wants to stall when I step on the accelerator, so I turn it off to avoid more damage (or at least until I can diagnose the problem). However, there is a very loud "squeak squeak squeak" sound coming from near the bell housing. It happens whenever the engine is on, even without driving. I have a 4L80-E transmission. Prior to hearing this sound, I noticed that it took a while for the truck to pick up speed. I've learned these could be symptoms of a bad torque converter.
I'm not sure if it is just the torque converter that needs to be replaced, or if the entire transmission is damaged (trans-oil is almost black). If anyone could give me advice about this, it would be greatly appreciated. I'm new to the forum so this is my first post.

Thanks!

Adam B.

If fluid is black drain it and look for metallic debris on magnet in pan if chunks of ceramic are present or if fluid smells like burnt rhubarb then a rebuild is needed. And i would replace torque converter and flush cooler lines both ways to flush out any debris in cooler and line sets till they come out clear clean flush fluid.

The other thing to check is engine mounts or any item that is rubbing the firewall

Will do!
Thanks a million Hardtop!
Godspeed!

Respectfully,
Adam B.

One other thing that crossed my mind is torque converter bolts or flex plate bolts make sure they're tight.