I've noticed similar to Braxus. I call it the "electric sound", because it sounds like the electric-like motors they used as sound effects for the van during the start of the movie "Bicentennial Man". I haven't thought it's a big deal, and I'm the opposite of many here -- I usually have my stereo *off* when I'm driving. Of course, I compose music professionally, so most of the time when I'm driving I'm working out a theme in my head, so it's very difficult to do that while listening to someone else's work
After the car is warmed up, the sound is less pronounced, though still present.