I had this problem with previous versions (now using 4.04) and couldn't resolve it so with a vey sore head gave up and recorded vocals outside of live.
It seemed to disappear with the first new versions of 4 but has popped up it's buzzy head again in 4.04 arrrrrgh!!!!
Is it possibly something to do with incorrect routing? which I find is a little tricky.
It's a constant buzz that starts when a track is armed for recording.
Any help would be much appreciated
thanks
Ian