Activate the fan by adjusting the thermostat setting. Observe the blades to make certain they are turning freely. Narrow the troubleshooting by listening from where noises may be coming. Vibration, dry parts, damaged bearings or faulty components are usually the causes of noise.
Look under the gable fan to see if metal shavings or metal pieces have been dropped by the fan. The presence of these elements usually predicts a large problem, like a worn-out motor, and may precipitate replacing the motor or the entire fan assembly.
Turn the fan off and disconnect power to the unit at the breaker box. Wait for the blades to come to a complete stop, then turn them by hand. You are looking for anything that might be catching the blades. Listen for any grinding or scraping noises as you turn.
Lubricate the motor shaft, louver hinges and the hub that drives the fan blades. Use a spray-on lubricant and/or add grease to the Zerk fitting if one is available. Zerk fittings are sometimes threaded onto components that periodically need grease applied as a lubricant. Zerks are about 1/8-inch tall, about 3/16-inch in diameter and tapered in the middle. They have a flat top with a small steel bearing, which is spring-loaded and acts as a valve to allow grease in and keep it there.
Check the drive belt if your fan has one. A loose or worn belt could be stretching so much that it causes a noisy fan. Cracks, pits and frays indicate replacement is needed. Deflecting the belt more than an inch with a finger could mean the belt is stretched. Adjust the tension if possible or replace the belt.
Check the mounting screws that hold the fan to the gable. Tighten any loose screws. Adding sections of self-adhesive felt or rubber washers between the housing and the gable can also dampen some noises.
Turn power back on to the fan after making sure tools are removed from the fan and nothing was left behind. Turn the thermostat so the fan will come on and listen for noises. If none, you've fixed the problem. If noises persist, it may be necessary to replace the attic gable fan.