If it is firing with no batteries, then that is odd...? You should hear the click, but the release should relies on separate part to capture and hold for the magnet to time the release. If you are looking to get inside, look for the rails and the armature that rides on it. It sound like an issue there. If the rails are clear, look at the small tab along the mirror cage. If it is clogged...
...then the shutter does not come to rest and the system does not reset...forever repeating the firing sequence.
Below is the other actuator that might be having problems.