tonight i noticed after hooking up my sterio system that i have a humming noise coming from my motor when i turn the key on (but the cars not running)..its not real loud but enough to notice it and my volt gauge is reading 12 also when the cars not on..is any of this normal..i took off my power wire that i hooked up and the noise is still there so i dont think it could be anything to do with the sterio system...any ideas?