I found it by accident, and I don't know if it was mentioned before, but if you change the sound preferences of your VM file, and choose "null" for the input sound device,
then Parallels stays on 9% of proccessor usage when idle, and on 2% when paused, otherwise it's on 18% idle and 10% paused, at least on my Core2Duo MBP.
then Parallels stays on 9% of proccessor usage when idle, and on 2% when paused, otherwise it's on 18% idle and 10% paused, at least on my Core2Duo MBP.