Computer fan blowing: blowing air, going all woosh. Can be used as a generic ventilation sound. Recorded using audacity, mono zingyou zy-ud1. Can be looped.
Recording of a bladeless fan or ventilation system in a hall at saxogade, vesterbro. The recorder is close to it. Feel free to download and use it, but remember to credit me. (rl).
I recorded my air conditioner sound recently. It's one of those thermostat style ones, not the window attachment one. This was recorded right on the vent taking in air as well.
Recorded in a big garage near an old air conditioning unit using the zoom h2n. The recording was slightly edited in adobe audition with the stereo expander effect.