Hi All, when I have my central heating on, everything is fine, all rads get hot and the boiler is efficient and quiet. However, I get a wierd bubbling noise which works its way around the system, from rad to rad. The only way I can describe it, is that it sounds like several large air bubbles (similar sounding to large mud bath bubbles - blub blub blub type of thing)are being pushed around the system, but surely if this was the case, it would just cause an airlock in one or two rads and stay static? I have tried bleeding every rad to no avail, and have also completely drained and refilled the system, so any other suggestions please? 
No other recent work has been done on the system, which is a normal header tank, gas fired potterton boiler. Just these noises, which have been happening for a couple of months.
No other recent work has been done on the system, which is a normal header tank, gas fired potterton boiler. Just these noises, which have been happening for a couple of months.