Ah, but often, as the compressor is more suited to bleeding air than an engine, the APU can provide higher pressure air, so in some regional types, the noise and flow rates are a lot better than putting engine air bleeds on whilst idling on the ground.
APUs are serviced based on cycles, hours and condition, plus a few older types suffer from oil weeping past seals and causing air quality issues. As Ground handling agencies charge like the Light Brigade for Ground power, airlines often prefer APU use to save costs, particularly with short turnaround times, but SOPs are increasingly turning towards turning APUs off immediately after main engine start and either sitting with airs off, or on engine air.
Also, some aircraft use electric starting engines, (turboprops 146/RJ) and others air starts. In either case, you'll hear the air conditioning, then it goes quiet, as you start to push back, and likely once the start is complete, the noise will change as you taxi out to take off.
Hope that helps.