Diagnosis
The noise is likely a foreign object contacting the fan blades or a failing fan motor bearing.
How to Fix It
Power off and unplug the unit. Remove the front cover and the PECO-Filter. Visually inspect the fan compartment for debris. Use a flashlight to check for small objects. Remove any debris. Reassemble and test.
If That Didn't Work
The fan motor assembly itself may be defective. The bearings can wear out, causing a grinding sound. This requires a replacement part from Molekule.
Heads Up
Do not attempt to lubricate or disassemble the fan motor. It is a sealed unit. Forcing it voids the warranty.