Recently my CH system started making "gurgling" sounds as though air is rushing around the system. I bled all radiators which made no difference. Today I decided to drain and refill the system and went to loft to tie ballcock up and found the header tank contained very hot water. I drained and refilled and then bled the system but still have sme problem. I have turned the pump setting down from 3 to 2 but still same. The bathroom radiatoe which is nearest to pump is filling with air and I can bleed quite a lot of air out of this daily. I fitted an old cast iron radiator to the system about 18 months ago but this has been fine. The system is still performing as in heating rads and hot water but the noise and hot water in header tank is causing me some concern. The system is approx 20 years old. Please help.