- the expansion tank could be shot. It should be mostly full of air. If it sounds like it is full of water, it's probably shot.
- the autofill valve (if you have one) is leaking, and needs to be replaced
- the manual fill valve (if you don't have an autofill valve) is leaking, and needs to be replaced
- and maybe a fourth - if you have an indirect WH or a hot water coil, it could be leaking internally, and the potable water system is pressurizing the boiler circuit.