Diagnosis
That's almost always a loose grille or a foreign object like a small piece of debris bouncing around on the fan blades. The fan assembly is pretty tight from the factory, so it's rarely an internal mechanical failure.
How to Fix It
First, unplug the heater and let it cool completely. 1) Check the front and rear protective grilles. Give them a gentle wiggle—sometimes a plastic clip doesn't seat fully after moving it. Press firmly around the edges to snap it back in. 2) Tilt the heater side-to-side and listen for anything sliding. If you hear it, look through the grilles with a flashlight. You might be able to see a twist-tie, piece of packing foam, or a bug. Use a can of compressed air (like for keyboards) to blow through the rear grille and dislodge it.
If That Didn't Work
If the noise is more of a rhythmic scraping or grinding, the fan motor shaft might need a drop of light machine oil. There's a small access port on the motor housing at the rear. One drop of 3-in-1 oil into the port can quiet a dry bearing. Don't overdo it.
Heads Up
Never try to bend the grille wires or insert tools to stop the fan—you can easily throw the blade out of balance or damage the heating element. If the noise is truly internal and the compressed air trick doesn't work, it's time to contact support.