Inside the bath fan, there is a motor to control the fan blade. The fan blade pulls air from the room and expels it out. Old models tend to be noisy. Additionally, cheap options use low-quality parts to keep the price low. That means, old models and low-cost options are cost-effective but make a loud noise while running. The vent fans that use a 3-inch duct have to work much harder to push air through the small hole that causes louder sound.