The buzz is indeed a "feature", though there are ways to work around it, all of them having a disadvatage unfortunately.
Apparently it is related to what stress you put on your cpu. For example, if I put energy saver on highest it is quite noticeable in a silent room. However, if I put it on automatic you really have to pay attention. In reduced mode it's almost not audible. So you can get the sizzle away, but at the expense of cpu power.
Another solution is to download CHUD from the apple website and through the hardware preference pane turn off NAP. If you do that even under highest energy saver setting tthere will be no sizzle left. The disdvantage is that your fan will come on far more quickly (apparently NAP is also an energy saving mechanism), and your battery will drain far more quickly.
So what I do is a. either work in automatic mode for non CPU-instensive tasks, or b. when I need the CPU power I put off NAP.
I really think apple should fix this. It is not super annoying, but when I am working at night (which unfortunately happens a lot) it really can get on my nerves, especially because it is not a constant sound but slightly irregular, like baking bacon or so.
Anyway, it's not a malfunction but a "regular" feature ...