Hissing from coolant expansion tank ford fiesta?

the other day when i was driving steam appeared from under the bonnet and it was coming from my coolant expansion tank, i slowly opened the cap and released the pressure when i got home i topped it up with coolant and water but after i drove it i could hear a hissing noise, ive bought a new cap but it still continues to make the noise the system is not leaking coolant and the temperature gauge stayed on normal. i was wondering if maybe its a water pump problem not circulating the fluid quick enough which is allowing it to boil.