There are several possible causes of your vent hood making noise, including:
Loose screws or bolts that may be rattling in the fan motor
A fan motor that is out of alignment or has been improperly installed
A worn-out fan blade and/or wheel, resulting in unusual vibration from the unit
Fan blades hitting against the vent hood guard, creating a loud noise
Air rushing through a clogged filter, creating turbulence within the unit