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

This one needs a licensed pro

Shut it off: a loose, broken or failing part

Screeching, squealing, banging, clanking, grinding and unexplained rattling all point to a part that is loose, broken or wearing out. Turn the system off and call an HVAC technician.

Why this happens

Lennox says strange sounds such as clanking, banging or squealing could indicate a serious problem that calls for immediate attention from an HVAC professional, and that it's best to turn the system off while you wait. Trane says a high-pitched noise from an outdoor unit could be related to the fan motor, capacitor or compressor. Carrier says grinding or clanking can mean a broken fan blade. Trane says a rattle with excessive vibration is not normal and can come from the blower wheel, a fan, the compressor, or a loose screw or bolt. American Standard says grinding, squealing, banging or rattling often mean loose parts, motor issues, electrical failures, or dirty or frozen coils.

What to do right now

  1. Set the thermostat to Off right away. Lennox says it's best to turn the system off while you wait for a technician, and Trane says to turn off an outdoor unit that makes a high-pitched noise to avoid damage.
  2. Keep it off. Carrier says not to run the AC if a fan blade is damaged, because a cracked or loose blade can damage the motor or coils.
  3. Leave every panel and the fan grille in place. Don't reach in, and don't try to tighten, oil or straighten anything inside.
  4. Write down what the sound was like, which unit it came from, and whether it happened at startup, while running or at shutdown.
  5. Call an HVAC technician.

When to stop. If you can see a cracked, bent or loose fan blade through the grille, keep the system off. Carrier and Trane both say to leave it off and call a professional.

What to tell the HVAC technician

  • What the noise sounded like and whether it came from the outdoor unit or the indoor unit.
  • When it happens: at startup, while running or at shutdown.
  • Anything you saw through the top grille, such as a damaged fan blade or something caught inside.
  • Whether the cooling changed when the noise started.
  • The brand and model from the outdoor unit's data plate, or from the label on the outside of the indoor unit. Don't open a panel to find it.

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

Start this troubleshooter over

References (every fact above comes from one of these pages)
  1. Lennox: Five ways your HVAC system is trying to get your attention
  2. Trane: Is this normal? HVAC smells and sounds, explained
  3. Carrier: AC fan not spinning
  4. Trane: AC fan not working
  5. American Standard: Top 10 signs you need air conditioning repair

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