Diagnosis
Excessive noise on high speed is often a loose or misaligned fan blade. The centrifugal blower wheel can also collect grease and wobble.
How to Fix It
Remove the grease filter and check the blower wheel behind it. Clean any heavy grease buildup with a degreaser. Ensure the wheel is seated firmly on the motor shaft.
If That Didn't Work
The damper flap on the exterior vent may be vibrating against the duct. Inspect the external vent cover. Ensure the damper moves freely and isn't stuck.
Heads Up
Always disconnect power before reaching into the vent area. The blower wheel is plastic; do not force it.