If the cooling system isn't under pressure, the coolant will boil and you will have to keep topping it up. If the expansion cap is hissing them it isn't holing pressure and the cap needs replacing. Don't let the tank boil dry, else you will need to bleed the cooling system to prevent air locks when refilling with coolant.
If you still find your loosing coolant with a new cap fitted, then its likely that either you have a leak from a point where the coolant is burning off rather then dripping onto the floor (inlet manifold gasket or water pump for example) or its leaking into the engine (identifiable from emulsification or 'mayo' on the dipstick and underside of the oil filler cap. In either case, taking it to a garage for a pressure test to identify the reason for the coolant loss is a good idea.
If you still find your loosing coolant with a new cap fitted, then its likely that either you have a leak from a point where the coolant is burning off rather then dripping onto the floor (inlet manifold gasket or water pump for example) or its leaking into the engine (identifiable from emulsification or 'mayo' on the dipstick and underside of the oil filler cap. In either case, taking it to a garage for a pressure test to identify the reason for the coolant loss is a good idea.