If your kitchen thermostat is your only t-stat then yes you are wasting enrgy. The rest of your home will feel extremely hot while the heat tries to get to the thermostat in the kitchen. The water that you hear is probably from air that is trapped in the heating water system, a full hydronic heating system makes no noise. Try bleeding the one remaining radiator in your kitchen to eliminate the sound. As to the ability of the one radiator to heat your kitchen, that depends on the size of the kitchen and radiator as well as the construction of your home, ie. wood, brick etc.. and the insulation factor.