Your HVAC system shouldn't sound like a haunted house. If you're hearing new or unusual sounds, each noise typically points to a specific problem. Here's what the most common sounds mean and how urgent each one is.
Banging or Booming
Urgency: Medium-High. A loud bang when your furnace turns on usually means delayed ignition — gas builds up before igniting, causing a small explosion. This can crack your heat exchanger over time. It could also be expanding/contracting ductwork, which is less urgent but still worth checking.
Squealing or Screeching
Urgency: Medium. A high-pitched squeal usually means a worn blower belt or failing blower motor bearings. The belt is a $100–$200 fix. The motor is $300–$700. Don't ignore it — a seized motor can cause overheating.
Clicking (Repeated)
Urgency: Medium. Repeated clicking when the furnace tries to start usually means a failing igniter or a problem with the flame sensor. The system is trying to ignite but can't. Igniter replacement runs $150–$300.
Rattling
Urgency: Low-Medium. Rattling often means loose screws, panels, or ductwork connections. Check for loose access panels on the furnace first. Persistent rattling could indicate a loose blower wheel or failing motor mount.
Humming or Buzzing
Urgency: Low-Medium. A loud hum from the outdoor AC unit often means a failing contactor, bad capacitor, or compressor issue. A buzzing indoor unit may have electrical issues. Capacitor replacement is $150–$350.
Whistling
Urgency: Low. Whistling usually means airflow restriction — most commonly a dirty filter. Replace your filter first. If it continues, you may have undersized ductwork or closed dampers.
Grinding
Urgency: High. Metal-on-metal grinding means something has broken loose inside the blower assembly. Turn the system off immediately and call for service. Running it in this state can cause extensive (and expensive) damage.