In the background of all my microphone recordings, especially the dialog recordings, is a soft whirring of case fans. You might not notice it's there, until I start editing files and laying them overtop one another, when suddenly the noise multiplies, bursts noticeably in and out of existence, and generally drives me crazy. In dialog for the mice, which is sped up, it's even more noticeable. Sensitive condenser microphones are necessary to avoid the way dynamic microphones sound obviously like someone's speaking into them - fine for characters who are supposed to be performing onstage, but not naturalistic. These mics pick up everything in the room, in greater detail than dynamics. Fortunately and unfortunately.
One anti-noise strategy is to employ a gate, a piece of hardware, or in my case a plugin, that cuts off the audio unless it's above a certain volume. You can twiddle settings to set this threshold, determine how quickly the gate opens and how quickly it closes again, and whether it looks ahead in the audio file to be ready to open when a loud signal is approaching. This is the method I think I've settled on; you don't really notice the noise if it only happens while people are speaking, and it eliminates the noise during silence which makes editing less of a headache.
For a while I attempted a very fancy workaround using Reaper's ReaFIR plugin, with which the computer built a noise profile for the background noise and then subtracted that from the sound. It worked miraculously, but also produced weird metallic-sounding artifacts in the output. So for now, I'm back to finessing the settings on the gate, and it works pleasantly.