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Likely cause: a clogged filter or closed vents
A whistle or a louder rush of air at the vents usually means the air is being squeezed somewhere. Open the vents and check the filter.
Why this happens
Goodman says that when the air filter or supply vents are blocked, or too many registers are closed off, the pressure and speed of the air can change and may raise the sound level. Trane says a high-pitched sound at a furnace can come from a dirty or too-restrictive filter. ENERGY STAR adds that a dirty filter can raise energy costs and damage your equipment. Goodman also says loose or ill-fitting ducts, or debris in the ductwork, can echo sounds through the registers.
How to fix it
- Walk the house and open the supply registers. Move furniture, rugs and curtains away from the supply vents and the return grilles.
- Turn the system off at the thermostat, then slide the filter out and look at it. Trane says to turn the HVAC system off before you remove the filter.
- If the filter is gray, dusty or clogged, replace it. Trane says the size should be listed on one or more edges of the old filter, and the arrow on the new one should point toward the furnace or air handler.
- Trane says a dirty filter, or one that is too restrictive for your system, can cause a high-pitched sound, and that a new filter is the fix when the noise stops with the old one out.
- Set the thermostat back to Cool and listen at the vents through a full cooling cycle.
When to stop. If what you hear is a hiss, not a whistle or a rush of air, stop, go back to the questions and pick Hissing. Trane says hissing is not a normal sound. If the vents are still noisy with a clean filter and every register open, call an HVAC technician. Goodman says unusual noises from the registers mean the ductwork should be inspected.
If this does not fix it, tell the HVAC technician
- Which vents are noisy and whether it is a whistle, a rush of air, a rattle or an echo.
- Whether a new filter and open registers changed the noise.
- What kind of filter you use and when you last changed it.
- Whether the noise started after a filter change or after vents were closed.
Build the full list: what to tell the HVAC technician.
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