Heat Pump Making Noise in Winter: Normal Defrost Sounds vs. Problem Sounds

Most heat pump noise in winter is the defrost cycle. Trane says you may hear a swooshing sound as the refrigerant reverses, the outdoor fan may stop, and a cloud of steam may rise, and Carrier's and Bryant's owner's manuals add a “whoosh” at the start and end of defrost, all normal. Grinding, rattling, screeching, banging, or a loud buzz is not: Trane says those sounds can mean electrical issues or system failure, and that you should always contact an HVAC professional.

Find your sound below. If the outdoor unit is caked in ice, see heat pump frozen; if cool air comes from the registers, see heat pump blowing cold air; for an indoor furnace, see furnace making noise.

Normal Sounds in Winter

The defrost whoosh. To melt frost off the outdoor coil, the reversing valve switches the unit into cooling for a few minutes, Trane explains, and you may hear a swoosh as the refrigerant reverses. Carrier's and Bryant's manuals put the whoosh at the beginning and end of defrost, and Lennox calls it normal. Carrier says to call only if it becomes excessively loud or constant.

Steam and running water. Steam or fog during defrost is normal (Carrier, Bryant, Lennox, Trane). Daikin says a water-flowing sound during heating is defrost, not a fault, lasting about 3 to 10 minutes.

The fan stops, then a hum. The outdoor fan stops during defrost (Bryant, Trane). Rheem's manual says the fan and compressor shut off at the start, and the compressor restarts about 30 seconds later to melt the ice; at the end, the fan restarts and the compressor pauses about 30 seconds again. Trane says heat pumps get louder in defrost, with a hum while the fan is stopped.

Louder on the coldest days. Carrier's and Bryant's variable-speed manuals say the system runs at higher speeds as it gets colder, and the higher speed produces a higher sound. Carrier adds a slight “purr” at start-up and shutdown, and Bryant a slight hiss in the off-cycle as pressure equalizes, both normal.

How long. Lennox and Bryant put a typical defrost at about 5 to 15 minutes; Trane's demand-defrost cycles usually take 3 to 5. Trane says defrost noise should settle within a few minutes.

Signs It's Not Normal

• Noise that doesn't settle within a few minutes, loud noise outside defrost, or a quiet unit that suddenly gets loud (Trane).
• Banging, screeching, rattling, buzzing, grinding, or gurgling, or any extremely loud or prolonged sound (Trane).
• A reversing-valve whoosh that is excessively loud or constant (Carrier).
• Defrost that comes too often or runs unusually long (Lennox). See heat pump running constantly.

Problem Sounds and What They Mean

1. Grinding. Lennox and Bryant both say to call a dealer for grinding. Goodman names loose parts, bearings, or motor components, and bearings that need lubrication, among noise sources. All are technician repairs.
2. Rattling or vibration. Trane says a rattle with excessive vibration is not normal and can come from the fan or fan motor, the compressor, a loose screw or bolt, or an outdoor unit that isn't level. Goodman adds debris, a fan blade that needs adjusting, and piping rubbing against something; Daikin says loose fan blades and failing bearings rattle. In heavy snow and ice storms, Carrier's manual says buildup on the top and fan blade can cause excessive vibration that may cause damage.
3. Screeching or squealing. Trane lists a loose screw, the fan motor, the capacitor, or the compressor behind a high-pitched sound from the outdoor unit, and says to turn it off and call for service. Lennox names squealing among the sounds that call for a licensed technician.
4. Banging or clanking. Lennox says clanking, banging, squealing, or prolonged hissing can indicate a serious problem that calls for immediate attention by an HVAC professional. Lennox also lists ice blocking the fan blades as a sign of a frozen unit.
5. Loud buzzing or humming. Trane says a loud hum from the outdoor unit can be the capacitor, contactor, fan motor, or something else, and calls for repair; it lists buzzing among the sounds that can mean electrical issues. Carrier says a distinct hum from an outdoor unit usually means a motor trying to start, and warns that capacitors can store lethal energy even with the power off, so it strongly recommends leaving them to a licensed technician.
6. Hissing with weak heat. A slight hiss in the off-cycle can be normal on variable-speed units (Bryant). But Trane says hissing near the outdoor refrigerant lines may be a large refrigerant leak, and to shut the system off and call. Lennox pairs hissing and reduced heating with low refrigerant.

What You Can Check Yourself

1. Time it. A whoosh, steam, and a stopped fan that end within about 15 minutes are defrost (Lennox, Bryant). Otherwise, note when the sound starts and how long it lasts; Goodman says that helps a dealer find the cause.
2. For a problem sound, switch it off. Turn the heat pump off (American Standard, Lennox). If it has an emergency heat setting, Carrier's manual says running on auxiliary heat with the heat pump off may be appropriate until snow and ice clear from the top and fan.
3. Cut the power at the disconnect box near the outdoor unit before you clear anything (Lennox).
4. Clear debris, snow, and ice from the outside. Look through the grille for leaves or sticks without reaching in. Carrier says to keep debris and snow from blocking airflow, with about 24 inches of clear space as a general guide, and Mitsubishi Electric says to keep the unit clear of snow and ice. Brush off loose snow; if ice coats the coil or fan, follow heat pump frozen and let it melt.
5. Look for loose panels from outside. Without removing anything, see whether a panel is missing a screw, sits crooked, or buzzes against the cabinet, and whether the unit tilts on its pad. Trane lists a loose screw or bolt and an unlevel unit among rattle causes; Bryant's manual says to re-level a settled unit promptly. Tell the technician what you see rather than opening the cabinet; Carrier says not to remove panels or work on internal components.
6. Check the filter and vents. Hold the filter up to the light; if you can't see through it, Lennox says replace it. Goodman lists a dirty filter and blocked vents among noise causes. Our filter guide shows the swap.
7. Check the breaker once. If the unit stopped and a breaker tripped, Bryant says you can reset it once; if it trips again, leave it off and call. See breaker keeps tripping.

What Not to Do

• Don't open panels or touch electrical parts (Lennox, Carrier). Trane says to get a professional for your safety and the heat pump's.
• Never chip ice. Carrier and Bryant say not to chip ice off the coil, and Lennox warns that tools can damage the coils, fins, and fan blades.
• Don't keep running it through a sudden loud or high-pitched noise; Trane says shut it off and call.
• Don't keep resetting a breaker that trips again (Carrier, Bryant).

When to Call a Pro

Bryant says a qualified technician diagnoses problems with refrigerant, electrical components, controls, airflow, the reversing valve, the compressor, and the defrost system. Call a licensed HVAC technician when:

• You hear grinding, rattling, screeching, squealing, banging, or loud buzzing (Trane, Lennox, Bryant).
• The reversing-valve whoosh is excessively loud or constant (Carrier).
• Defrost noise doesn't settle within a few minutes, or a quiet unit suddenly gets loud (Trane).
• You hear hissing near the refrigerant lines, or hissing with weak heat (Trane, Lennox).
• Defrost comes too often or runs unusually long (Lennox).
• Ice blocks the fan, or the fan won't spin after the ice melts (Lennox).
• A breaker trips again after one reset (Bryant).

If you'll be away while you wait, keep the heat no lower than 55°F, the American Red Cross minimum, so the wait doesn't end in frozen pipes.

Prevent It

• Keep it clear. Bryant's manual says not to let grass clippings, leaves, or debris collect on the sides or top, and to keep at least 12 inches from tall grass, vines, and shrubs. Mitsubishi Electric says to keep its surface clear of snow and ice.
• Keep it level and raised. Bryant's manual says units in snowy areas sit on support feet and should be re-leveled if the base settles. Mitsubishi Electric says contractors should mount it above the snow line.
• Change the filter. The Department of Energy says monthly during peak seasons, and to clear weeds and debris from around the outdoor unit.
• Book maintenance. Goodman says regular maintenance by a licensed dealer may prevent noise-causing malfunctions, including checking airflow, inspecting the outdoor fan, and lubricating moving parts. ENERGY STAR recommends a pre-season check of the heating system in the fall. The winter guide schedules it.

Frequently Asked Questions

Why does my heat pump make a whooshing sound?

Usually it's the reversing valve switching for defrost, which Carrier's and Bryant's manuals call normal at the start and end. Carrier says to call if it becomes excessively loud or constant.

Why is my heat pump louder in cold weather?

Carrier's and Bryant's manuals say variable-speed models run faster, and so louder, as it gets colder, and defrost adds a few louder minutes (Trane). New grinding, rattling, or screeching is different.

Is it safe to keep running a noisy heat pump?

Not for a problem sound. American Standard says turn it off completely and call an HVAC professional immediately, and Lennox says turning it off while you wait can keep a small issue from becoming a bigger one.

The Bottom Line

A whoosh at the start and end of defrost, steam, a stopped fan, and a hum that settles within a few minutes are normal. Grinding, rattling, screeching, banging, loud buzzing, or a hiss near the lines is not. Turn it off, cut the power, clear debris and snow, look for loose panels without opening anything, check the filter, and call a licensed HVAC technician.

Sources

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