With Summer fast approaching I've turned my A/C on a few times and noticed this piercing whistling/whining noise from my A/C. When I first turn on the unit there is no noise but after about 1.5/2 minutes the noise kicks in. When I switch the A/C off the noise stops immediately. The noise appears to become more apparent the cooler the outside temperature is. The noise happens at both the condenser and evaporator side. I asked this question elsewhere and was told to block the condenser fan for five minutes to see if the noise changed. I have done this twice so far and each time after a few minutes the noise goes away just about completely. As you would imagine after unblocking the condenser the noise comes back after a few minutes. I was told this means the low refrigerant. If that is the case would that also mean I have a leak? If that's also the case would would be the next step? Have a tech refill and run some leak stopper product? The unit (Heil) looks ancient and if I were to guess I would say it's from the 80's. I can't find the model or anything on Google. Thanks for the help.