Boiler making loud noise?

I had no hot water this morning although the CH was working fine. Nothing came out of the hot water tap in any rooms. In the afternoon i switched off the boiler from the mains and back on. After running the tap the hot water came back but 5 mins later the boiler was making a loud clunking/banging noise. I switched it off and then back on but it immediatly came back. Tried it 1 hr later and it seems to be running fine now but worried ot leave it on. Engineer cant come out till 2 days.
Could it be that there was some ice in the system and it had'nt melted or maybe something actually broken?. I have a Ideal Sprint Rapide 90nf combi boiler which is quite old but seems to run fine up to today.
The pressure gauge reads between 0 & 1 but not sure if thats what it always is.