ive got a 92' 525i E34, just finished replacing the thermostat, went to start the car, it sounded weak but it started, ran it for a couple of minutes then turned it off and tried to start it again, the engine turns over maybe three times then theirs a loud rattling sound like a machine gun. all the lights and radio turn on but the car wont start. the cars been sitting for a couple of days could it be a weak battery?