Car makes strange noise after taking out the key. It sounds like a pomp is working and pumping fluid around. That results in an empty battery the next day. Strange, but this is not happening every day.
It sounds like you might have two problems here. I'm thinking maybe the fluid noise is caused by a low coolant problem. When the coolant gets low it creates air in the system and then whey you stop the car you can hear it moving around.
As far as the battery being dead you should have it tested to see if is a good battery.