Diagnosis
That's almost certainly the blower fan wheel hitting something or its bearings failing. Debris gets sucked in, or the wheel's hub cracks over time from vibration.
How to Fix It
Unplug the unit. Remove the front grille and the air filter. Look inside at the large, squirrel-cage blower wheel. Shine a light in and slowly rotate the wheel by hand. You're looking for anything touching it—a clump of insulation, a stray wire, or a piece of plastic. Remove any debris. Also check if the wheel is loose on its shaft; it shouldn't wiggle side-to-side much.
If That Didn't Work
If the wheel looks clear and is secure, the motor bearings are probably shot. The fan motor is a separate component. You'll need to disconnect it and replace it. The part number is often stamped on the motor housing.
Heads Up
Don't run it with that noise for long. A badly unbalanced fan wheel can damage the motor mounts or even cause the motor to overheat. It's a fix you want to tackle sooner rather than later.