For me the following few steps have solved the problem every time I've had it:
1. Switch gear / computers / monitors to different circuits (if it's possible - if you're like me, you're in a house built a century ago and there is only one circuit for everything and you have to unplug the fridge, the light in the garage, the aquarium and the neighbor's LED-porch-light-extravaganza in order to get a decent recording)
2. Get a ground lift, but don't be silly about it - be careful with your expensive gear (you've probably hardly paid the interest on it and that insurance scam only works once). http://en.wikipedia.org/wiki/Ground_lift
3. Use a ground loop isolator/isolation transformer
4. Attend way too many techno shows until that particular range of hearing is severely damaged for a reasonable amount of time. (Refine your optimum ratio of time enduring bad
techno / duration of the damaged hearing - do not listen to (you know who/what) for one hour, only to gain a mere five minutes without that awful ground loop)
Sorry for the jokes - spent the night reading on homeoboxes. It's not what you think, unless you're thinking it is what it actually is.