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 is 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 freon pressures equalize, during this period sometimes sounds are 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. They 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 from the outdoor unit when it's not even 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 blade 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 put a hole in it causing the freon 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. Sometimes it is in the unit itself and cannot be eliminated.

Below is a list of possible causes.

Low refrigerant charge - can cause "gurgling sounds"
Bad reversing valve - passes freon internally, makes "hissing sound"
Bad compressor valves
Bad motor
Out of balanced or broken fan blades
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 out of balanced fan blade
Vibration due to loose parts.
Vibration due to piping strapped too tight