What are the best noise reduction plugins? I suppose it doesn't have to be open source, but that would be nice.
Noise reduction is normally best achieved in dedicated stand-alone applications. You should take a look at gnome wave cleaner (or gwc for short).
Another option, depending on the type of the noise and the wanted signal is simply gating and/or eq'ing the signal. Gate to remove noise from places where there is no signal and eq to attenuate the noise throughout the signal.
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