During the colder months when defrost mode switches on it can run for roughly ten minutes to thaw out the unit. I understand that. My question is why is it so loud? Why does it have to make such a loud noise to defrost the system? Is it not possible to make a defrost cycle more silent?
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.
What you're hearing is the reversing valve changing position. The reversing vale as the name implies reverses the flow of refrigerant when the valve changes position. When that happens a loud whoosh sound can be heard which may be what you're hearing.
I just read your comment about the loud buzzing. That is the compressor. Sound blankets exist to cover the compressor and help with deadening the noise.
Units with an inverter driven compressor in addition to a sound blanket operate extremely quietly. You are interested take a look at Mitsubishi's ductless units sound specs