Grinding/Screech from Block

[RhapidFyre]

Hey all, back once again but this time the truck is street...street...well.... uh, street-somewhat. It runs and can get from A to B. With that being said!

I have this interesting issue where there's a grinding/screeching noise, like an old belt noise coming somewhere from around the block/behind the block. I've taken the belts off to ensure it's not something that's being turned, or the belts themselves. Still screeching.

I have no idea how to relate the noise to anything except possibly a loose bearing. It sounds like a wheel turning and grinding against something else? I don't have a video right now to showcase it, but imagine the noise that a loose fan with a bad bearing would make, until I can get a video.

The noise does not increase with RPMs/Acceleration.
The noise is constant.
Gear selection does not change anything. Noise occurs in P/N/D.
Engine Temp/Oil Temp doesn't affect it - Hot or Cold, noise occurs.
Noise occurs immediately after startup.
Noise does not exist when turning over.

1969 GMC Custom 1/2 ton
Chevy 350 small block, new plugs, new belts
Edelbrock Carb (1406?) - 500CFM (New)
Truck drives with no noticeable issues.

[EDIT]
SOLVED
Power brake plug in the back side of the Edelbrock was open. Once plugged, the sound stopped completely.
Will be hooking the brake booster hose to it appropriately soon.