Are you using the Spectral Repair part of your Izotope Rx ?
Pulling out specific frequencies as opposed to the broadband 'wash' often means you get less artifacting for this sort of noise.
Or, if you have the waves Q10 it's amazing what you can do with a tight, tight bandwith to remove specific frequencies. If they're constant it's really easy... if they move.. it's a bit trickier. As Shaun says they'll be based around whatever your power runs at (50hz, 60hz) but could be any of the harmonics (multiples) above that number, 60, 120, 180, 240 etc.
... in response to your comment:
@Chris, ah ok, I think the 'hum' in the subject line might be at odds with the kind of noise you're dealing with. Have you tried some fast downward expansion in the upper ranges? If you have the C4 or ML4000 try setting the top two bands of the expander to a range of eg.-6db and set your thresholds so that when there is no dialogue there is -6db of expansion, but when there is dialogue it is opening up to 0db. Have the attack as fast as you can, and with the release find the line between following the decay of the words naturally while avoiding 'blooming' where you can hear the noise tailing out.
It's amazing how much you can trick the ear simply by turning things down. This will avoid the artifacting you're talking about but maybe introduce other problems... (take a listen to exterior scenes on tv shows that have obviously not been ADR'd and you can often hear the ext noise moving with the dialogue.)
Lesser of two evils?