ikon 28t boiler fault?

i have a ikon 28t boiler gc 47 407 12 and the safety lockout light keeps illuminating when i run the hot tap the boiler makes a banb bang bang noise then locks out if i reset it it then works briefly the the same thing happens shortly afterwords iv bled the heating system and the pump topped the boiler up with water there are no visible leaks what coulb it be?