Humming sound: the sound of the APU generator OR the GPU. Electrical systems on aircraft operate at 400 Hz. I tested this one day to confirm during work on the ramp with a scope on my phone. As long as the plane has electrical power, which is pretty much as long as the door is open (cuz what would anyone do on a dark plane? Lol), you’ll hear this. Battery won’t be switched on unless there is standby power available, otherwise you’re discharging the battery with no trade off.
Hissing sound/whir: Recirculation fans. Only works as long as the plane has electrical systems running. Usually used in conjunction with the yellow tube we hook up to the fuselage. Make sure to put the probe on the jetway inside the cabin somewhere otherwise you’ll fault the AC unit and the tube will blow unconditioned air which feels miserable lol.
As long as either the BATT, APU GEN, or EXT PWR is on, you’ll hear this stuff. Once the yellow tube is disconnected, you’re looking for the APU BLEED and PACKS to be switched to ON to provide cool or warm air for the cabin.
One last thing… louder humming sound, usually closer to departure time: that’s the hydraulic system being actuated by the ground crew (or pilots up front), given they have to switch on an auxiliary hydraulic system, such as in the MD-80. The Airbus also sometimes uses an AUX HYD system when at the gate, and it will make a woo-woo sound. I think MX usually is the one using it. Note, this is NOT the PTU I’m talking about, which makes that characteristic barking sound.
Source: I work around and on Airbuses and MD-80s about 35-40 hours per week.