This is the sort of problem that is best fixed at the source because AC noise has a very broad, uniform spectrum. Ideally you want that AC off while recording.
If this is not possible, the next best thing at the recording stage is to set your microphone to cardioid so that it picks up sound mainly from the front, and position it so that its back side is facing the AC. (I had a look on its manual to see what rejection you get on the back side but to my surprise I didn't find a polar diagram.)
If you put headphones on, you can hear how turning the microphone (always on the cardioid setting) will change the level of the AC being recorded and you can pick the position that works best. This will have a lot of effect on its higher frequencies but the lower ones will still be heard. But at least you should get 5-10dB less AC noise and centered at lower frequencies.
To improve on that, you could try and control the direct reflections by placing baffles like Thomas mentioned. Of course the feasibility of that depends of what you are recording but since this is a library I think I can safely assume it's not a drum set. Heavy blankets or duvets are common household items that can do this for cheap.
The general idea is that you're trying to block, either by microphone placement, or by absorption, the transmission of sound from the AC to the mic and higher frequencies are always easier because they are more directional and easily absorbed. This way you are pushing the noise to lower and lower frequencies until (hopefully) you can get it to a point where you can use a low shelving (if you have some content you want in that range) or low cut (if you don't) to get rid of it.
The other thing to keep in mind is signal to noise ratio or SNR. In your case, this is the sound you want to AC noise ratio since the AC will dominate the system noise. To improve this ratio, you need the mic as close to the source and as far from the AC as possible BUT don't place whatever you're recording (and the mic) in a corner because that will actually increase the noise at low frequencies and make your recording 'boomy'.
The reason this is important is because when you turn the gain up (either from the volume control on the mic, or in software), you increase both the level of the sound you want and the AC noise.
An ideal distance for a single voice is about 5cm. Any closer and you will get other problems like popping and breathing noise and the voice will lose legibility because of the proximity effect. Of course if you are covering multiple voices (or instruments) with the same mic, you won't be able to place it so closely, but recording is all about making the best compromise.
After the recording, there are many ways to treat noise but which one works best depends both on the material and the noise.
- Gates can remove noise between louder sounds and work best when you have a signal that has sharp transitions
- Expanders can amplify sounds above the noise floor and can work really well for continuous noise, even when the transitions are not as sharp
- Equalizers can reduce noises that are centered around one frequency band like whistling noises
- Noise reduction tools that are usually specialized and often quite expensive can be used for the rest of the cases.
If you have to resort to a noise reduction tool, I would suggest one that takes a 'fingerprint' of the noise and removes it. This would work best for the AC noise since it is continuous and doesn't change in pitch or level so the noise print would be very simple and effective.
