Your Home Comfort System, over its life span, could make a variety of noises.  Any noise out of the ordinary should be a reason for concern, and therefore you should notify Mills Heating & Air as soon as possible.  One sound you should concern yourself with is a high-pitched whistling sound.  It could be an indicator of leaking refrigerant.  It can damage the unit and also be a severe health issue for your family.  The refrigerant is a tasteless, odorless gas and can cause breathing problems and fluid buildup in the lung if exposed to it long enough.  If you notice this sound, turn off your unit immediately and call Mills Heating & Air to have the issue resolved by one of our trained professionals.