96 Maxima losing water?

My car has been running hot lately. The plastic jug thing that holds water also, however, never empties. the radiator is about 2 years old. Since then i still have to add water but not as often as i have used to have to, but now its like every other day. And when I open the radiator cap to add water, the plastic jug thing gurgles and makes a racket (this is new. ) So what are your thoughts? Water pump?? Thermostat?? Please help