The problem is that, no matter how much you try to remove noise sources, some will still creep into your recordings. e.g.,
To address this, drag your cursor over a part of the audio that is a good representation of a noisy area which should be quiet, (called the "noise profile"); this displays in a darker gray:
Select Effect > Noise Reduction, then Step 1: Get Noise Profile:
Next, select a single point anywhere in the audio, so as to deselect the noise profile:
Select Effect > Noise Reduction again, then Step 2: Apply a 12dB noise reduction with sensitivity 6, smoothing 0, reduce noise, and OK:
There you go: Nice and silent!