I am renting a house with a new Heil ( FSM2X300A) heat pump system. It does a good job cooling but one thing I have noticed is it will randomly blow warm air,I measured 76.5 going into the air return and and 75 at the ceiling vent.It seems to do this for a duration on average of 15 to 20 minutes then a loud hiss can be heard from the air handler closet, and the air will begin to cool again. It will then cool will no problem until the same cycle happens again. it happens very randomly but seems most frequent in mid afternoon. I have already had a tech out, he said everything was fine and changed the thermostat but I have the same thing going on. The owner of the property lives in another state and has asked me to not let problems go unattended, and I'm worried this will develop into something serious. There is obviously a problem and I want to get it addressed while the unit is still under warranty. Thanks in advance for any knowledge you can pass on.