HI. The most common issue here will be with a loose motor shaft. This is the part that the fan blades are mounted on. When this occurs, the looseness will cause the motor, and shaft to wobble, thus, causing this noise you here at high speeds. The wobble is less violent during the slower speeds, so it is not loud, in this case.

The second issue will be a warped fan blade. This will cause the fan to rub against the shroud, thus, creating the noise, as well.

If the shaft is loose, or bent, the entire motor will need to be replaced. Now, if the fan blade is warped, you can, simply, unscrew the blade from the shaft, and replace it(on most model)

These motors are real junk, i've had 3 in 2 years.

Go to www.broan.com and scream at tech support.

These units tend to be noisy. Make certain the duct work (if ducted) is installed as recommended. No severe changes in diameter, hard turns right at the top discharge, etc.
If air is restricted at the fan inlet or outlet, then the fan will turn slightly faster, usually creating more noise.
The way the hood is mounted to the cabinets could also be a source of noise. Putting rubber washers between the hood the the cabinet will reduce the transmitted vibration, when done right.