Q: Heard Popping Sounds After Key Removal

Today I was doing a coolant pressure test. I know I was supposed to disconnect the battery beforehand but I've had issues with the terminals connecting properly in the past so I left it alone because I figured the vehicle was off and hadn't been driven in hours. I disconnected the two sensors attached to the air intake and removed the air cleaner assembly box so I could get a better view of my hoses. At some point I got in and turned the key (not to start the car) just to turn the lights on, but I immediately noticed my heater was on and blowing (the sensors and assembly were still disconnected at this time). I quickly turned the heater fan off and turned the key back to the off position and removed it from the ignition. At that point I heard 3-4 very quiet popping sounds coming from inside the dash behind the steering wheel. My question is, do you think the quiet popping sound was related to not bleeding the coolant system yet, an electrical short, or just air sounds?

My car has 130000 miles.
My car has an automatic transmission.