Exhaust fans help ventilate the air in a room; in areas such as the kitchen or bathroom, they not only help pull the heat out of the room, but also moisture build up that could otherwise cause mold. Lubricating the fan's motor and even the fan blades can help keep it quiet while it does its job.
Before doing anything, turn off the circuit that sends power to the exhaust fan for safety. When the circuit is off, make sure the area around the vent is clean. If there is a lot of dust or dirt built up in the vent, the fan will just pull more dust and dirt onto itself. Most vent covers are held in place by screws or springs. Covers held in place with springs usually have tabs that can be pressed to compress the springs. Clean the vent cover with cloth and a cleaning solution. The type of product to use depends upon how dirty your exhaust fan is; most household cleaning products will work for light amounts of build up, but you may also need degreaser for kitchen exhaust fans.
Inspecting the Fan
If you lubricate a dirty/dusty fan, the lubricant is less likely to do its job. To clean the fan blade, remove the fan from the motor. The fan is either held in place by screws or brackets that can be compressed. You may also need to unplug wires from the fan to fully remove it. While you're looking at the wires, inspect them to make sure they're still good. Worn wire coverings, places where they are bent, or a corroded wire coverings are signs that the wires should be replaced. Mark where each wire is connected so you remember where to put them back later.
Clean the fan with a cloth and cleaning solution. Go over both fan blades as well as the motor itself.
Lubricating the Fan
Locate the place where the shaft of the fan connects to the motor. After ensuring that this area is as clean as possible, put a few drops of lubricant on the shaft of the fan. WD-40 is a good lubricant for exhaust fans because it will also help break up any remaining dirt, dust and grease, although you can also use most other industrial lubricants to lubricate an exhaust fan. Use your hand to spin the fan blades a few times, and clean up any lubricant that drips down the motor. Reapply lubricant if the fan doesn't spin easily; if it spins OK, reconnect the motor wires to the fan and place it back in the vent. Spin the fan a few times again to make sure it isn't touching the edge of the vent and that it spins easily.
Regular cleaning and lubricating will help the exhaust fan last longer, in addition to reducing the noise it makes.