Why is the defrost cycle of an HVAC so loud?
The defrost cycle is basically just running the heat pump in cooling mode without the outdoor fan. It's not going to be any louder then when it's running in cooling, it's just that the compressor noise is  disguised by the outdoor fan. The same holds true when the heat pump is in heating mode. The same compressor runs and knows no difference between heating, cooling or defrost mode.