The short answer

Noise is a clue about movement or resistance, not a direct measure of airflow. A fan can sound powerful while a blocked grille, crushed duct, stuck backdraft damper, or attic termination prevents moist air from leaving. Inspect the route before selecting a replacement fan.

Sort the sound

Symptom Possible area Useful next observation
Rattle at startup Grille, flap, or loose housing Does it continue after the motor settles?
Deep hum Motor, bearing, or restriction Does the grille draw tissue steadily?
Whistle Gap, damper, or narrow duct Does wind change the sound while off?
Weak draw Blockage, duct sag, or dirty grille Is the exterior flap opening?
Condensation Cold duct, attic discharge, or moisture load Where is the first wet point?

Turn the fan off before cleaning or inspecting the grille. Do not reach into a duct or attic from an unsafe position. If the duct terminates in an attic or wall cavity, treat that as a routing problem rather than a fan-noise problem. Ask for the full route, exterior cap, support, insulation, and electrical work to be listed separately.

Improve the service conversation

Record when the sound occurs, whether it changes with wind, how long the fan runs after bathing, and whether the room stays damp. A new grille or motor may be appropriate, but it cannot correct a crushed duct or a backdraft path. The replacement should preserve safe access and a proper exterior termination.

Limits

This guide does not size a fan, approve an electrical change, or determine local venting code. Persistent dampness, attic staining, or unsafe wiring needs qualified assessment.