I know what this noise it, I have it too. It happens when I turn on something that draws a good amount of power. It comes from the drivers side of the engine bay and almost sounds like a screaching, not a squealing from wet belts, but something else. I also use Nokya Purple 90 watt 9006 headlight bulbs. It only happens when I turn them on. The noise does go away when I am over 1500RPM. Is this what's happening to you as well? When I am at an idle (750RPM +/- 50) I will drop slightly below 12 volts and the noise will start. I think it's coming from the alternator, it's possibly just being stressed a little bit at low voltage with the constant draw from the headlights. When I turn my headlights off the noise doesn't completely go away, but its significantly lower. Lemme know if your getting the same thing!