Loud banging sounds from central heating

Alvis

Around last year, our central system was starting to screw up really badly. It made these really loud banging noises, as if someone is kicking on the radiators. The banging usually happens once or twice a day; at very random times when the heating had been switched on for a while. So at first I suspected there was an air bubble somewhere in the system. And I suggested to my parents that we drain the whole system to eliminate the air bubble. However, they were not comptempt with doing that (due to the sheer amount of effort involved), and also since I couldn't find where the drain cock was for the whole circuit. There was one, but that was mounted above the boiler which is mounted on the wall. So turning that drain cock would only drain the radiators upstairs, not downstairs. We left it alone for a while, thinking it the air bubble would just gradually move up to the top and burst. However that didn't happen. Over the next few months the water pump thing failed. So now it was banging as soon as the heating was switched on.

We got some guy from Bord Gais to replace it with another one. The banging stopped. Then it had started once again after another few months. Once or twice a day, the radiators would bang when all the radiators had been heated for about half and hour. Only happens when its gotten pretty hot already. The boiler sounds like its stalling and stuff and it's quite scary.

So last week we got another dude to arrive and he figured the boiler is overheating so it bangs. He replaced the low-limit thermostat and also the high-limit thermostat (I think that's what they were called). Needless to say, it didn't work. And now its even louder than before.

What on earth is causing this? Where is the drain cock that's below the water level of all the radiators to be found? Also do boilers corrode easily on the inside? Btw, I don't live in an area where there is hard water. The water here is very soft.

Thanks for reading.