So my AC/Heat has been making a really weird noise. It sounds like a mechanic whirring and it’s aggressively loud. This morning I get in my car and the noise is gone…but so is the heat. It’s not working at all and my windshield isn’t defrosting (it was frosted over). I go back about an hour later with a friend and he checks the coolant and he says it’s at the optimal level. I then turn back on the car and it’s bam the heat is working now and that noise is back. What could it be?