Five Common Causes
| Cause | Sign | Fix |
|---|---|---|
| Clogged or wrong-size filter | Whistle at the return or furnace | Fit the right size, change it |
| Closed or blocked registers | Whistle at other open vents | Open them, move furniture |
| Blocked or small return | Doors pull shut, return roars | Clear it; a technician checks sizing |
| Bent register fins | Whistle at one register | Straighten or replace the register |
| Leaks at joints or boots | Whistle in walls, ceilings or the attic | Seal with mastic or metal tape |
Checks in Order, Cheapest First
Check the Filter
A clogged filter or one that doesn't fit the slot forces air through gaps. Match the size printed on the old filter.
Open Every Register
Closing vents in unused rooms raises pressure in the ducts and pushes air out through leaks and the remaining vents.
Clear the Returns
Furniture, rugs or curtains over a return starve the system of air.
Listen at the Walls
A whistle from inside a wall or ceiling points to a leak at a joint or boot.
Closed registers also cause the hot and cold rooms described in the uneven temperatures guide.
Rattles and Bangs Are Different
A rattle usually means something is loose: a register screw, a damper, or a duct section that moves when the blower starts. A bang when the system starts or stops is often thin sheet metal flexing as pressure changes. Both point to a loose or unsupported duct, and the ductwork repair page covers the common fixes.
When to Call a Technician
If the whistle survives a new filter and open registers, the cause is inside the duct system. ENERGY STAR estimates typical ducts lose 20 to 30 percent of their air to leaks, and a whistling joint is one you can hear. Duct leak detection finds the leaks, and sealing with mastic or metal tape quiets them.