It's a rhythmic hum, oscillating in pitch from low to high. Had it last winter, completely disappeared in the spring/summer/fall. Now that the temps are below freezing it's back. Comes from the driver's cylinder bank and just by popping the hood and looking around it seems to be coming from the head of the cylinder bank. As the car warms up, the hum dissipates but yesterday it was really cold and the hum was present the whole commute home (about 25 minutes).