Noise coming from the vents themselves — not the furnace or AC unit — almost always traces back to the ductwork carrying air between them and the room. Here’s what different sounds usually mean.
A noise that’s new or localized to one vent usually means something’s shifted in the ductwork. Worth a look from air duct repair before it gets louder.
Banging or Popping
Often caused by thin sheet metal ductwork expanding and contracting as air temperature changes rapidly — most common right when a system starts up or shuts off. Occasional, brief banging at startup/shutdown is usually harmless; frequent or loud banging can mean a duct section has come loose or is flexing more than it should.
Whistling
Almost always airflow squeezing through a gap that’s too small — a nearly-closed damper, a mostly-blocked vent, or a duct that’s undersized for the airflow being pushed through it. Try opening the vent fully or checking for an accidentally closed damper first.
Rattling
Loose vent covers, a duct panel that’s worked its screws loose, or debris (sometimes literally a piece of insulation or drywall) caught inside a duct run vibrating as air passes.
A Low Humming or Roaring
Can indicate air moving through an undersized duct or one restricted somewhere along its length — the system is pushing harder to move the same amount of air.
A Metallic Clicking as the System Cycles
Usually the same thermal expansion/contraction as banging, just a smaller-scale version — normal in most systems, especially older metal ductwork.
When It’s Worth Investigating
A noise that’s new, that’s gotten noticeably louder, or that’s localized to one specific vent or room usually points to something changed — a duct coming loose, a damper shifting, or debris that’s moved into a new spot — worth a look rather than assuming it’ll resolve on its own.