I just installed a new radio in my car and I noticed that when the car is "on" but both radio and car is off, I can hear a buzzing sound that sounds like it's coming from the AC control (or something just behind it).

It's not the power steering sound which is louder and higher frequency. Anyone know what this is? is this normal? Or have I created an electrical time bomb?

 

If you have the auto a/c, there is a tiny fan in the right bottom 'vent' in the control that runs anytime the ignition switch is on in the accessory or run positions. That is normal. It seems to get louder over time as the bearings in the fan get worn. If it's considerably louder than previously, make sure you dont have any wiring pressing against the unit from behind. That will distort the housing and make it louder. Put your finger over the vent and see if the pitch changes.