whistling noise

i was driving home tonight, and i noticed a whistling sound coming from my car. i thought it might be the head unit picking up engine noise, but the sound continued when i turned the head unit off. i stopped my car pulled over and got out. the whistling couldnt be heard outside. it could only be heard when i was inside. its driving me nuts! its this annoying high pitched whistle! it started whistling while the car was idling but now it does it constantly. the noise seems to be coming from the dashboard. i have my tach wires running through there, could they possibly be the reason?