The moring in cold the heater on 4

At 7:15AM, when I go to school in college. I stop at the traffic light with all dense fogs on front window. Then I put 4 speeds for heater to removed all the dense fog. The sound say "BOOM!". Something wrong with the car with the heater on 4 speeds of A/C. I better to check it out the a/c. Maybe the frost on the engine are hard to warming up or the fan speeds.

Anyone know what the sound BOOM? from the heater on a/c to removed the fog windows.