Diagnosis
The noise is from the motor bearings. On these dryers, the bearing lubrication dries out or the bearing itself wears, causing metal-on-metal friction.
How to Fix It
This requires motor disassembly. Remove the rear grille and motor housing. Extract the motor assembly. Apply a single drop of high-temperature synthetic bearing oil (like Super Lube 51004) to each end of the motor shaft. Do not over-lubricate. Reassemble and test.
If That Didn't Work
The bearing race is likely scored. The motor is a sealed unit. You must replace the entire DC motor assembly. Part number is typically on the motor casing.
Heads Up
Never use WD-40 or 3-in-1 oil. They attract dust and will gum up, causing the motor to fail completely from overheating.