DIAGNOSIS Something is loose and being vibrated by the fan. The usual suspects are the removable pan under the basket, the basket itself, or the fan's protective cover inside the unit.
FIX 1) Unplug the fryer and let it cool. 2) Remove the basket and the square crisper plate inside it. Now, pull out the black, perforated interior pan from the bottom of the cooking chamber. 3) Reseat this pan firmly and flatly. It must sit perfectly level. Then reassemble the basket and try again. The noise is often that pan being slightly askew.
IF THAT DIDN'T WORK The fan cover inside the top of the fryer might have loosened. You can sometimes see it by looking up into the heating element area. If a screw is visibly loose, you can carefully tighten it with a long Phillips head, but if you're not comfortable, don't risk it.
HEADS UP A high-pitched whirring is normal fan noise. A grinding or scraping sound is not—stop use immediately if you hear that, as the fan might be hitting something.