Just bought a 2002 ranger that has a 3.0 motor and 5 speed transmission. It's making a grinding metal to metal noise that I have never heard before. Any ideas what it could be. I'm in the process of removing the inspection plate on the transmission. Also removed the serpentine belt to eliminate those items in the front of the motor.

 

Welcome to the forum

You have a 3.0l M5OD-R1 transmission, AKA M5R1, these transmissions are engine specific, i.e. 3.0l M5R1 will not fit on 4.0l or 2.3l and visa versa
Also 2WD or 4x4 specific
The M5R1 uses ATF, yes automatic transmission fluid in a manual transmission, Ford's Mercon V is spec
Using "gear oil" will cause transmission failure

Go to top of a small hill that you can roll down with engine off, remember you will lose Power Brakes after the 3rd pedal press, so may need to restart engine
While rolling with engine off and with clutch pedal pushed down/held down, shift in to 1st gear then 2nd then 3rd..........., also try Neutral with clutch pedal up
See if there is a specific noise in specific gear, or gets louder in certain gear, and if noise increases and decreases with speed

Any manual transmission's gears can not be disconnected from the rear wheels, only from the engine via the clutch, so rolling with engine off tests the gears and input shaft inside transmission

And with engine off the flywheel/ring gear is also not spinning, so that eliminates starter gear rubbing on ring gear

While not common the M5R1 input shaft can fail, known failure but not at all common, more common on the 4cyl Rangers
Generally caused by failed Pilot Bearing or input shaft bearing