Noisy central heating

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.

You could try the following:-

(1) Take a look in your airing cupboard and see if there are any bleed valves in the supply pipework to the hot water cylinder. If you can find any, try loosening them and see if any air escapes.

(2) Whilst you are there, close off the pump valves and remove the pump - have a bucket to catch any water. Now hold the bucket under the top pump valve and open it up. You should now have filthy water pouring into the bucket. If this isn't the case then you have a blocked cold feed and it starts to get a little difficult. You have two basic options, either clear the blockage or cut out the blocked pipework and replace with new.

Good luck.

Checked in the airing cupboard, there are no valves. The pump is located above the boiler, not near to the cylinder. The pipework to all the radiators and to the cylinder gets hot so does this mean that there is not a blockage. I have seen on other posts that a de-aerator has been recommeded, would this help in this case?