Vent, Valve & Volt
AC making a loud or strange noise

This is normal

Probably normal: everyday operating sounds

A soft hum, the whir of the outdoor unit, the usual start and stop sounds and a ping from the ducts are part of normal running. Nothing needs fixing.

Why this happens

Trane says a heating and cooling system should not make odd noises beyond humming or its normal startup and shutdown sounds, and it counts ductwork pinging as it expands and contracts and the whir of the outdoor unit among a home's normal sounds. American Standard also ties a ping to ducts contracting and expanding. Goodman describes a working air conditioner's sound as a soft hum. Trane adds that some air conditioners and heat pumps are naturally loud when starting and stopping, and that a heat pump hums with its fan stopped during defrost mode, which is normal.

What to do

  1. Keep using the system as usual.
  2. Get to know its regular sounds so a change stands out. Trane says changes in the startup and shutdown sounds are important.
  3. If a sound changes or gets louder, write down what changed and when. Goodman says noting new sounds can help an HVAC dealer find the probable cause.
  4. If a new sound shows up, come back and run through these questions again. If the system suddenly gets loud, set the thermostat to Off first. Trane says to shut the system off and call for repair.

When to stop. If a sound changes, such as a hum that turns into a buzz, a rattle or a screech, or the unit becomes noticeably louder than usual, call an HVAC technician. Goodman says a significant change in sounds may be a signal to have the system inspected, and Trane says a unit that gets noticeably louder often points to a failing capacitor or compressor. If it suddenly gets loud, Trane says to shut the system off.

If it still worries you, tell the HVAC technician

  • The sounds the system normally makes and what has changed.
  • Whether the startup or shutdown sound has gotten louder.
  • Whether the cooling has changed along with the sound.

Build the full list: what to tell the HVAC technician.

Related troubleshooters

Start this troubleshooter over

References (every fact above comes from one of these pages)
  1. Trane: Is this normal? HVAC smells and sounds, explained
  2. Goodman: Why does my neighbor's air conditioner sound loud?
  3. American Standard: Noises and smells from your HVAC system

FourHome is our sister brand. Browse all FourHome replacement faucet and shower parts on the FourHome Amazon store. We earn from purchases made through this link.