The pressure on my boiler keeps going down and needs constantly topped up?

Once a day the pressure falls to 0 and needs to be topped up to 1.5. There are no obvious leaks at any of the radiators that I can see and no leaks under the boiler. When the pressure is correct the heating works fine although the hot water is very unreliable now. The boiler is making a hideous sound like it's very dry and it's coughing.