Where I work we have continuous air flow. However, you can't hear it at all. It's not even a background noise it's so quiet. I mean, you really have to sit and focus your attention to hear it, and even then it's just barely there. The only time you even realize there was sound is when the power goes out and the whole building makes that sudden sigh into silence (all the computers going off, the air, the light, all the inperceptable hum vanishing...)