Loose screws, a bad fan motor or a damaged fan blade can cause loud fan noises on a range hood. First, tighten any loose installation screws that hold the range hood to the wall and cabinet. Check the fan mounting screws and tighten any loose screws.
Check the fan blade for damage. Replace the fan blade if it's broken or damaged. Worn bearings in the fan motor can make a loud squealing or whining noise and the fan motor typically gets extremely hot when motor bearings are bad. Replace the fan motor if it squeals or whines and gets extremely hot to the touch while running.
