Why does my LG Microwave range hood hum loudly when the fan turns on?
In my range hood there was a loud hum when the fan was on, and it turned to be a metal screen that was slipping and brushing against the running fan. The fan blades scraped along the metal grill, causing the noise.

I would look inside the hood if you can, look to see where the fan is, and make sure that nothing is loose or disturbing the fan's motion. Also check that all the screws and plates are tight - loose plates can hum when they vibrate.