Outdoor unit makes strange/loud noises:

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 "wooshing sound", which usually lasts for a couple of 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. And 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, or 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 causing 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.

Below is a list of possible causes. Items in red usually require a service call. Items in blue however can be addressed, some even fixed by the homeowner.

Red = Professional fix | Blue = Homeowner fix

Bad compressor valves
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

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

The bottom causes in blue are commonly found problems and can be addressed by the homeowner. Try to check for these conditions first before calling for service.

Commonly Reported HVAC Problems