Diagnosis
A foreign object is interfering with the fan blades, or the fan blade itself is out of balance or hitting the housing.
How to Fix It
Unplug the heater. Tilt it front-to-back and side-to-side while listening for something loose rolling inside. Use a flashlight to look through the front and rear grilles for debris. Use long-nose pliers or tweezers to carefully remove any visible object.
If That Didn't Work
The fan motor shaft bearings may be defective. This requires disassembly. Remove the rear grille to access the fan assembly. Spin the fan blade by hand. If it grinds or has lateral play, the motor needs replacement.
Heads Up
Never use compressed air to spin the fan while the unit is plugged in. It can generate a current and damage the control board.