It is happening all the time, though I don't alway notice is, and then when I do, its maddenly distracting. Its like in the range of frequencies used for hearing tests when you go to the ear doctor, if that makes sense.
I thought at first it was Google Voice (as it there's an extra connection made in between you and the recipient), but dialing straight to the recipient without GV as an intermediary still results in this faint noise. Model 43/09. I've had no reboot issues or othet things, though I do notice that the when I play through the ringtones on my phone and during navigation, there is a slight hiss following the tones - as if the tones and voice navigation words were recorded imperfectly.
EDIT: I made a phonecall just now, and there was no noise. Then 50 seconds in, the high pitched interference-like sound started and slowly and steadily lowered in pitch over the course of 40 seconds till it went a way. And then a little later did the same thing. Its very faint, but there enough that its a definite annoyance.