Many heat pump noises are normal: a whoosh or hiss during defrost, a click when it starts or stops, and a steady low hum while running. Noises that signal trouble include loud banging or clanking, grinding, screeching, constant loud buzzing, and rattling that won’t stop. If you hear any of those, turn the system off at the thermostat and check the outdoor unit for debris or ice. If the noise doesn’t have an obvious cause, call a technician before running it again.
Normal sounds vs warning sounds
| Sound | Usually normal? | Most likely cause |
|---|---|---|
| Whoosh, hiss or “steam” sound for a few minutes in winter | Yes | Defrost cycle switching the refrigerant flow |
| Single click or clunk at start or stop | Yes | Contactor, relay or reversing valve switching |
| Steady low hum | Yes | Compressor and fans running normally |
| Gentle gurgling or trickling | Yes | Refrigerant moving through lines; water draining |
| Fan swooshing that rises and falls (inverter models) | Yes | Variable-speed fan adjusting |
| Loud, constant buzzing | No | Electrical problem: contactor, capacitor or fan motor |
| Banging, clanking or thumping | No | Loose or broken part, debris, ice on fan |
| Rattling | Sometimes | Loose panels, screws or debris (often a simple fix) |
| Grinding | No | Worn motor bearings |
| Screeching or squealing | No | Failing fan motor, or high pressure in the compressor |
| Repeated clicking with no start | No | Failing capacitor, contactor or control board |
| Loud hissing or bubbling that doesn’t stop | No | Refrigerant leak |
What each noise means and what to do
Whooshing or hissing (usually during defrost)
In winter, your heat pump periodically reverses to melt frost off the outdoor coil. When the reversing valve switches, refrigerant rushes through the lines with a loud whoosh, the outdoor fan stops, and you may see steam rising. It lasts 5–15 minutes and repeats every 30–90 minutes in cold, humid weather.
What to do: nothing, if it matches that pattern. If hissing is constant, happens outside of defrost, or comes with weak heating or ice on the copper lines, it may be a refrigerant leak — call a technician.
Banging, clanking or thumping
- A single thump at startup or shutdown can be the compressor settling or the reversing valve shifting. Usually harmless.
- Repeated banging while running often means something loose or broken: a fan blade hitting the grille or ice, a loose compressor mount, or a broken part inside the compressor.
- Banging in ductwork (indoors) is usually metal ducts expanding and contracting as air pressure changes.
What to do: turn the system off. Check the outdoor unit for sticks, toys, ice or snow touching the fan, and look for bent fan blades. If you find nothing, don’t run it again until a technician checks it; a loose internal part can quickly destroy the compressor. Duct popping is not dangerous; a technician can add bracing or adjust airflow if it bothers you.
Buzzing
- A quiet hum is normal.
- A loud, constant buzz from the outdoor unit, especially when the unit should be off, often points to a stuck or worn contactor, a failing capacitor, or a fan motor struggling to start.
- Buzzing with the fan not spinning usually means the fan motor or its capacitor has failed.
- Buzzing from the indoor unit can be a loose panel, a failing blower motor, or loose wiring.
What to do: if the outdoor unit buzzes while the thermostat isn’t calling for heat or cooling, turn off power at the breaker. Electrical buzzing is a job for a technician. Don’t touch capacitors — they hold a charge even when the power is off.
Rattling
Rattling usually comes from loose access panels, screws, or debris (twigs, leaves, small stones) inside the outdoor cabinet. It can also be a refrigerant line vibrating against the house or another pipe.
What to do: with power off, tighten loose panel screws and clear debris from the top grille and coil. If a refrigerant line is touching a wall, a technician can add a rubber isolation pad. Rattling deep inside the unit may be a loose fan mount or compressor part.
Grinding
A metal-on-metal grinding sound usually means worn motor bearings in the outdoor fan motor or the indoor blower motor.
What to do: turn the system off and call a technician. Worn bearings get worse quickly, and a seized motor can damage other parts.
Screeching or squealing
A high-pitched squeal often comes from a failing fan or blower motor. A very loud screech when the compressor starts can mean dangerously high internal pressure.
What to do: shut the system off right away and call a technician. High-pressure screeching is a safety issue.
Clicking
One or two clicks when the system turns on or off are normal: relays and the contactor switching. Rapid or repeated clicking with no startup usually means a failing capacitor, a bad contactor, or a control board problem.
What to do: check the thermostat batteries and the breaker. If the clicking continues and the unit won’t start, call a technician.
Gurgling or bubbling
Light gurgling is refrigerant moving or condensate draining, and it’s normal. Loud, constant bubbling together with weak heating or cooling can mean low refrigerant or air in the lines.
Vibration and humming you can feel inside
If the compressor’s vibration travels into the house through the wall or floor, the unit may have lost its rubber isolation pads, or its pad or bracket has settled. A technician can re-level it or add vibration isolators.
When to call a technician
Turn the system off and call right away if you hear:
- Screeching or squealing, especially at compressor startup
- Grinding
- Loud banging or clanking while it runs, with no debris or ice to explain it
- Loud buzzing while the unit should be off, or buzzing with the fan not spinning
- Constant hissing or bubbling along with weak heating or cooling
If you smell burning or electrical odors, turn the system off at the breaker immediately.
While you wait, use Emergency Heat in winter to keep the house warm without running the outdoor unit.
Typical repair costs
Approximate 2026 US prices, parts and labor:
| Repair | Typical cost |
|---|---|
| Diagnostic visit | $100–$250 |
| Tighten panels, add vibration pads or line isolators | $100–$300 |
| Capacitor replacement | $150–$450 |
| Contactor replacement | $150–$400 |
| Outdoor fan blade replacement | $150–$400 |
| Outdoor fan motor replacement | $350–$800 |
| Indoor blower motor replacement | $450–$1,500 |
| Reversing valve replacement | $1,000–$2,500 |
| Refrigerant leak repair + recharge | $400–$1,500+ |
| Compressor replacement | $2,000–$4,500 |
If your heat pump is over 12–15 years old and needs a compressor or reversing valve, compare the repair price with a new, quieter system.
How to make a heat pump quieter
Even a healthy heat pump makes some noise. Most modern outdoor units run at about 50–70 decibels — roughly a conversation to a running dishwasher — and variable-speed models can drop into the 40s at low speed. If yours is louder than you’d like:
- Clean and tighten. Clear debris, rinse the coil, and tighten loose panel screws (with power off).
- Add vibration isolation. Rubber anti-vibration pads under the unit, or isolators on a wall bracket, stop vibration from traveling into the house.
- Check the pad. A cracked or sunken concrete pad can make the unit tilt and vibrate. Re-leveling is a quick job for a technician.
- Isolate the refrigerant lines where they touch the wall or floor joists.
- Turn on quiet or night mode. Many inverter models and smart thermostats have a low-noise setting that limits fan and compressor speed at night.
- Add a sound barrier. A fence, lattice screen or shrubs placed at least 2–3 feet from the unit can soften sound without blocking airflow. Never enclose the unit or block the top.
- Use a compressor sound blanket. Many manufacturers sell insulated blankets that wrap the compressor inside the cabinet. They need a technician to install, typically for $150–$400.
- Consider location at replacement time. Keep the new outdoor unit away from bedroom windows, patios and narrow side yards that echo noise. Variable-speed models are usually the quietest choice.
Frequently asked questions
Why does my heat pump make a loud whooshing noise in winter?
That’s almost always the defrost cycle. The reversing valve switches refrigerant flow to melt frost off the outdoor coil, causing a whoosh and often steam. It’s normal if it lasts 5–15 minutes and repeats every 30–90 minutes in cold, humid weather.
Why does my heat pump bang when it turns on or off?
One thump at start or stop is usually the compressor or reversing valve settling. Repeated banging while running is not normal; turn it off and check for debris, ice or a loose part.
Is it normal for a heat pump to buzz?
A low hum is normal. A loud, constant buzz, especially when the unit should be off or the fan isn’t spinning, usually signals an electrical problem like a failing contactor or capacitor.
Why does my heat pump make noise when it’s off?
Some units run a crankcase heater in cold weather, which can make a faint hum. A loud buzz when the system is off often means a stuck contactor, which should be checked.
How loud should a heat pump be?
Most outdoor units are about 50–70 decibels while running. Variable-speed models can run in the 40s at low speed. Check the decibel rating on the spec sheet when shopping.
Can I keep running my heat pump if it’s making a strange noise?
Not if it’s grinding, screeching, banging or buzzing loudly. Those can turn a small repair into a compressor replacement. Switch to Emergency Heat and call a technician.
Related guides
- Heat Pump Defrost Cycle: What Is Normal and What Is Not
- Heat Pump Frozen Outside: How to Defrost It Safely
- Heat Pump Short Cycling: Causes, Risks and Fixes
- Heat Pump Blowing Cold Air in Heat Mode: 9 Causes and Fixes
- Quietest Mini Splits for Bedrooms
- Heat Pump Maintenance Checklist by Season
This guide is general information. Turn off power before inspecting equipment, never touch capacitors, and leave electrical, refrigerant and internal repairs to a licensed HVAC technician. Repair prices are approximate 2026 national ranges.