Central heating noises

[PaulO]

I have a Potterton Netaheat Boiler and when the heating is switched on and also whilst it is running I experience loud banging noises from around the house. I am unable to determine exactly where they are coming from and the noises are not just at start up.
I have bleed the radiators (9) several times and although I often get air out of different radiators this fails to stop the noise. When I bleed the radiators the air seem to be present at different points at different times.
Incidentally should I bleed the radiators with the system off or on? and should I start at the nearest or furthest from the boiler and/or start upstairs or down?
I moved into this house about three months ago and the noise have been here since we moved in. I do not know if they were present before.
Sorry if this is not enough information, but let me know what you need and I will try to supply it.
Regards

 

[ChrisR]

Bleed with boiler/pump OFF. Sequence not critical.
Shouldn't need to keep on bleeding, add a corrosion inhibitor.
Try operating with boiler temp set lower.
Old boilers can thump a bit, banging in rads more likely to be due to pipes being jammed so they can't expand properly.

 

[PaulO]

I have also discovered a couple of bleed valves that are situated in the airing cupboard at the top of a short length of pipework.
What do I do with these?

 

[ChrisR]

V important! OPen them and let the air out, fairly obviously... If there was much in there though your HW wouldn't get hot.
My Netaheat isn't as quiet as it was - try turning its thermostat down - any difference?