it's kind of like a noise of shooting air
I suppose the sound you are referring to is the excess air being dumped overboard by the surge control valve. It happens when the pneumatic demand goes from high to low. I.E: air con packs off prior to eng start, eng start completed.
It could also be the APU air inlet taking in more air when the pneumatic demand goes from low to high. But I doubt so, as the former's "shooting air" noise is more pronounced.
If the surge control valve is not working, you will hear a more startling sound of "gun shots". It will help if you could pinpoint exactly when you heard the sound.