High pitch noise even when NO KEY in the ignition - any ideas??

It started happening this morning (it was freezing - I couldn't open my doors for a while). There is a faint, high pitch noise - like "e-e-e-e-e-e-e-e-e-e-e...." coming from *somewhere" under the hood. The strange thing is - it is there even when there is NO key in the ignition. It doesn't matter if the doors are closed or open. And it is not the same noise that you get when you push alarm button and the door is open. It sounds kinda like that but much more quiet. The only thing that makes it go away is disconnecting the battery. I have no idea what this is and I can't figure out where exactly the noise is coming from. Any ideas will definitely help. I am out of warranty and I'd like to avoid bringing it to my dealer. I am really clueless what the hell it might be... :confused:

Actually, believe it or not, it could be your battery venting. There may be a shorted cell and the gasses expand when there is even just a tiny load on it and it could be just enough pressure to cause a slight whistle. Do you have any discoloration on the top of the battery like dark spots?

Thanks for the tip, Scott. Couldn't find anything unusual with my battery. After leaving my battery disconnected overnight it seems that the noise is gone. Hopefully that will be the end of story. :rollo2: