I've just put a full charge battery in the back of my car and the instant it is linked up (no keys in the ignition) there is the noise of someone using a power drill coming from my engine bay. Im currently trying to get a video so i can show what i mean but the batteries of my video cam are flat at the moment

im going to try and find the source of the noise one more time but its bloody scary, it sounds like something's spinning 1000x too fast and is about to blow up!
any ideas?