water leak

Hi, i have a celica on an 04 plate that is constantly losing water when the engine is running. (There is no sign of any contamination in the oil or on the radiator cap) I havent taken the floor pan off as of yet but was after some thoughts first as i dont want to get on my back as i have no ramps.
When the engine is on tick over the coolant level seems constant. When you increase the revs the water seems to disappear and then comes back after the rad cap has been released. Also only when the fans kick in does it look like there is air in the system (you can see that as the water runs back to where the water breather is its all bubbly. I wasnt sure if this could be related to the air con? It is a seperate system isnt it? I have put so much water in her that there is no antifreeze in her and the leak seems to be of a green fluid type. So at this moment in time i am a bit lost! Any help would be much appreciated.