Well... having buzz introduced by heavily touted features is kind of bad. The bakers themselves very much acknowledged this and were quick to state it was a top priority for the D update. Disabling the 64 bit thingie as they suggested or rolling back to X3b (provided you weren't experiencing any of the bugs fixed in C) seem to be reasonable solutions. I did actually notice a few things between the brief time I was messing around with the b patch and how the c patch is responding. With the b patch things were loading LIGHTNING fast and everything just seemed far snappier. After installing the c patch though there was significantly more lag and that hyper snappiness cooled a bit... but in both cases no more than I experience with X2a and probably less actually.
So as I move forward with writing a new project (my first) in X3c I'm keeping an eye out for any problems. If things start going wacky I may uninstall and just patch up to b and see if that's better.
That said I do not get the hum everyone is talking about and I even tried turning on the CE to see if something would happen. It just isn't there. Granted I have very few tracks on this project so perhaps it's cumulative... or I could just be deaf and not hear it. I don't think so though.
One thing that MAY be a result of this though is on my newly programmed TH2 preset I'm having a bit of a hard time gating out some buzz before the gate closes but I think that is more due to the ultra distorted high gain settings of the set up as opposed to anything coming from Sonar. As soon as the gate closes it's gone.