Re: Super-clean audio - how do they do it?

Well, there may be several reasons for this, but from a production stand point here are three:

One: they may have still turned stuff off like the AC, refridgerator, fans, etc.

Two: Low-cut filters! If you don't know how to use them, you need to learn. Low-cut filters (located on field mixers) will eliminate frequencies below a set frequency, like 120Hz, 140Hz. This will rid you of low frequencies like the ever popular 60Hz associated with AC units and the like...sounds you can't hear with your ears but will end up on your recording.

Three: using the right mics. Good boom mics like the Scheops MK-41 and Sennheiser MKH 60 (and others known as "true" condensers) will give you the best sound possible. Yes, they're expensive, but well worth it!