Well, here is one I haven't heard before?

Last night when I pulled into my garage and turned off my engine, I heard a constant electronic buzzing sound coming from my drivers side dash board area. The sound continued with the key turned off and even pulled out!! I opened the door and it still continued, it wasn't stopping! I got back in and turned on the key, it still kept going, I then started the engine and it finally stopped.
I've had my car for 6 months and never heard this before!.

Anyone have any idea what it could have been??