Diagnosis
This is not normal. A very loud fan on high is often caused by incorrect ducting or an obstruction.
How to Fix It
Check the external damper on the wall cap. Ensure it opens freely. Verify the ductwork is straight, short, and uses rigid metal duct. Flexible duct causes turbulence and noise. The duct size must be at least 3 1/4" x 10" or 4" round diameter.
If That Didn't Work
The fan impeller may have been damaged or is contacting the housing. Inspect for debris. If clear, the fan motor assembly may be defective.
Heads Up
For vertical discharge installations, you must install the supplied vertical duct cover. Running without it can cause excessive noise and recirculation.