I am interested in getting a DSP card like UAD-1 or Powercore. I was very hooked on the UAD until I read about how many problems there are with current drivers, so I am waiting. Also, UA dropped the possibility of 3rd party developers making plugins for the platform, and new plugins are coming out from UA VERY SLOWLY, whereas several developers have now announced support for Powercore. So even though I am sure UAD-1 is the more powerful card, I am now not so sure what to get...
I got to your post, by looking for info on the UAD. I had the same noise problem - all of a sudden - with an Audiowerk2 soundcard a little while ago - I could monitor all of my G4 inner life. For some reason that has totally stopped and I hear nothing anymore, don´t ask me why. I only changed the external settings, i don´t use a mixer anymore, I go straight from AW2 outputs to powered monitors - and it´s quiet like before.
mabe check your connections and cables and try different connections.
ps: how do you like the UAD, any info on the Powercore?