1986 Chevy Nova loud medium pitched noise when car is off (

Hello,
I have a 1986 Chevrolet Nova. As mentioned it has approx. 103,000 miles on it. For a long time my horn didn't work. Now it does, but when my car is off it makes a loud medium pitched noise, much like a horn. Its coming from the front passenger side right near the headlight. It makes this sound with the car off and no keys in it. My horn now works, but this sound won't go away. It is seems like a different noise than the horn and is coming from a different place. When I unplug the battery it turns off. I plugged it back in and the sound went away for a while, but now its back on without the car running. It doesn't really flucuate in loudness. I did fiddle around with a few wires near my alternator (dont think they are part of the alternator but near them on the passenger side) and the pitch changed a little but then went back to normal. Someone mentioned to me my AC and heat motor. Pump or something. Is this a possibility? My air was blowing a little on the weak side. I would really appreciate a response. Thank you so much! SLL