2003 mcs 6 speed with 100k

I think it's the transmission and not the engine.
It sounds like a washing machine on spin dry at those RPMs.
It's been a couple months now, even with the noise it drives smooth.
Clutch is strong, although the clutch pedal is crazy squeaky like an old door.

Would like to know what to fix!

 

Does the sound go away if you put the car in neutral a speed....
Also can you make the car make the noise by reving it when parked?
Has the tires been balanced recently...a slipped belt can make a huge racket/thumping....
These questions might help folks guess at a few things for you to look at...still kinda long shot.