Heat pumps do tend to makes strange and/or loud noises at times, more so in the winter. Heat pumps have reversing valves that reverse the flow of refrigerant between the heating and cooling modes.
During the winter, whenever the heat pump goes into the defrost mode, this valve shifts. Along with that comes a "whooshing sound," which usually lasts for a couple seconds. After that, the compressor sometimes sounds much louder than usual, almost a "tinny sound." After shutdown the refrigerant pressures equalize, during this period sounds can sometimes be heard, but this is normal.
Another common loud sound is when the outdoor unit starts up or shuts off. Specifically, the newer "scroll" type compressors tend to make a "back-peddling" type of noise on shutdown. On start-up, they sometimes sound like an "out-of-balanced washing machine."
Sometimes customers complain of a buzzing noise coming from the outdoor unit, even when it's not running. This is usually the reversing valve solenoid coil. It's low voltage (24 volts) so it isn't really wasting energy and sometimes they can be heard.
If you are hearing a very loud "metal-hitting-metal type sound," the fan blades could be hitting something; possibly ice, a wire or tubing. Take a look and shut the unit off immediately. This almost always ruins the fan blades and possibly the motor as well. If a piece of copper tubing shifted and is being hit by the blades, they could cut a hole in it and cause the refrigerant to leak out. Then there is always the vibration noise, which sounds simple but can be the most difficult to eliminate. Sometimes it is just a matter of installing rubber isolation pads under the unit. Sometimes the refrigerant piping is strapped too tightly to the joists or studs in the home. Sometimes it is in the unit itself and cannot be eliminated.
These items usually require a service call:
- Bad motor
- Out of balanced or broken fan blades
- Low refrigerant charge – can cause "gurgling sounds"
- Bad reversing valve – passes refrigerant internally, makes "hissing sound"
- Buzzing contactor or noisy solenoid coil
- Loud compressor
- Loud unit
These items can be addressed by the homeowner. Try to check for these conditions first before calling for service:
- Bad compressor valves
- Outdoor unit iced-up, fan blades hitting ice (weather-related)
- Fan blades hitting some other obstruction
- Vibration due to loose parts
- Vibration due to refrigerant piping being strapped too tightly