DIAGNOSIS A foreign object has likely been pulled into the fan blower wheel, or the wheel itself has come loose or has a broken blade. The fan is behind the lower rear grille.
FIX Safety first: unplug the unit. 1) Remove the bucket. 2) Look at the lower rear air intake grille. You'll see several screws holding it on. Remove these screws (a Phillips head screwdriver) and carefully pull the grille off. 3) You'll now see the large, circular plastic fan wheel. Shine a flashlight and visually inspect for any debris—twist ties, paper, insulation, etc.—stuck between the blades or in the housing. Remove anything you find. 4) Gently try to wiggle the fan wheel side-to-side. It should have very little play. If it's wobbling excessively, the shaft or bearing is damaged.
IF THAT DIDN'T WORK If the wheel looks clear and secure, the noise might be from the motor bearings. With the unit unplugged, try spinning the fan wheel by hand. It should spin smoothly and quietly. Any grinding or gritty feeling points to a failing motor.
HEADS UP Always keep the area around the rear grille clear of curtains, papers, or loose items. That fan pulls a lot of air and can suck in small objects, which is the most common cause of this noise.