Thanks Mettelus,
I was tinkering around with ReaFir last night and finally managed to get it to do what I need by using it in the FX bin of a track.
I would prefer to use it as a 'destructive operation' in Sound Forge so that my clip is forever clean - in my mind, this means less stress on SONAR as it won't need to be processing every time I play back. So far I haven't managed to get this working in Sound Forge yet - whenever I capture a noiseprint of a short section at the beginning of a clip, it gets overwritten before I can process the reduction across the whole track. I'll keep tinkering and googling on that one...
Of course I could use SONAR to create a noise-free processed duplicate clip and use this instead but I do miss the ease of popping over to Sound Forge and processing (in around 30 seconds as you mentioned in your comment about Audition earlier).
Regarding Sanderxpander's point about working out where the noise is coming from. It's actually pretty quiet in my small studio but I don't have expensive mics or means to buy them so, at the end of a track when there's 6 or 8 instruments fading out on their final note, the aggregated mic-hiss or amp-hum is JUST noticeable enough for me to want to kill it off. Using a free VST for clean-up is the best solution for me.