What is that Noise

I've been hearing a loud whistle sound that comes from the outdoor part of my home A/C system. It seems to cool just fine but makes that noise when it shuts off. It doesn't sound like a belt slipping but more like a high pressure gas release of some sort. Is this something I need to worry about? (can I fix it?)

Sounds like you have a heat pump that is equalizing when it shuts off this is not unusual but the noise being annoying is a concern, one possibility is that your head pressure is very high. You need to check your outdoor coil and make sure it is clean and your outdoor fan is running. Hopefully you haven't had service work done recently and the system got overcharged. The other possibility is the reversing valve is not coming back all the way when the solenoid releases, this could be a serious problem turn your system to heat and make sure your heat pump works all right, this may alleviate the problem but if not and your heat works you can decide to live with the noise or replace the reversing valve.