I haven't been able to find anyone online with quite this issue... I have a Carrier heat pump system, and through the winter it would often make that torpedo noise when heating the house (I later learned this is defrost mode to melt condensation that has frozen). It seems my AC system goes into defrost mode more than necessary, as I live in Florida and the temperatures are rarely below freezing, even at night. I do remember reading that humidity alters the freezing point somewhat. I also read there's a hack to get defrost mode to turn on only when necessary, rather than when the system "thinks" there is ice buildup.
Anyway, now that it's the hottest time of the year, I periodically hear a similar torpedo-shooting-like sound come from the outdoor unit. This happens several seconds (about 10 seconds) after the exhaust fan turns off. I have a Nest thermostat and so the air handler fan will run for a minute or so after the outdoor unit turns off. This noise is not made every time the outdoor unit shuts off, and seems to only happen at night, when my thermostat moves the temp from 78 to 74.
The only info I could find was that a spring was bad in the compressor, and it's a sealed unit so replacing it would mean replacing the whole outdoor unit. Is this a good use of my home warranty (it is covered)? Assuming this is something I can ignore, it is next to my bedroom window, and wakes me up at night.