Heating & Cooling · Common problems · Updated November 2026
Strange HVAC Noises and What They Can Mean
Every system makes some noise. A new or changing sound is worth paying attention to, because it can signal a part that is about to fail.
If you smell gas, or your carbon monoxide alarm sounds, leave the house now and call 911 or your gas utility from outside. If you smell burning or see smoke, turn the system off at the thermostat and the breaker if it is safe to do so.
What different sounds can mean
| Sound | Possible cause | What to do |
|---|---|---|
| Squealing or screeching | Worn blower or fan motor bearing or belt | Schedule service soon |
| Banging or clanking | Loose or broken part in the blower or outdoor unit | Turn the system off and call |
| Buzzing or humming from the outdoor unit | Electrical issue, such as a failing capacitor or contactor | Call; do not open the unit |
| Clicking that repeats without starting | Ignition, relay or control problem | Call |
| Hissing or bubbling | Possible refrigerant leak, or air in ducts | Call |
| Loud boom when the furnace starts | Delayed ignition | Turn the furnace off and call |
| Rattling vents or ducts | Loose duct or panel | Can usually wait for regular service |
What you can safely do
- Note when the sound happens: at startup, while running, or at shutdown.
- Check that the filter door and panels are closed.
- Clear sticks and debris from around the outdoor unit.
- Record a short video of the sound for the technician.
Do not open electrical panels on the furnace or outdoor unit. Capacitors can hold a dangerous charge even with the power off.
When to call a pro
Call for any banging, booming, buzzing or burning smell, or any noise that comes with poor heating or cooling.
Cost figures are national ranges from the sources listed in our heating & cooling cost guide, not a quote from FixFinch or any contractor. This article is general information to help you decide on next steps. It is not an inspection or professional advice; a qualified pro who sees the problem in person can tell you what is actually going on. Calls connect you with an independent service provider in our network. FixFinch is not a contractor and does not perform the work.