I recently drained the system and flushed it so I could put fernox in. Since then the system has made various noises from banging,whooshing,gurgling(when the heating comes in) and most commonly a sound similar to the noise you hear when a cistern is filling although the pitch is always slightly changing(this is all the time the hot water is on). I have tried to bleed the radiators several times but there is no air in them. I have switched off all the radiators and run the pump at it's highest setting to see if there is any air to be forced around. Today I put my hand into the expansion tank for the heating and could feel a gentle flow of hot water into the tank through the cold feed pipe. Can anyone give me some advice please. The system is a Gloworm boiler on a microbore system. (Don't know if this makes any difference but the hot water has to be on if the central heating is needed. No room stats.