hi,my central heating system has developed a very loud noise.it appears to come from the boiler which is in the garage below the bathroom.when i start the heating within 20 secs the noise ,which is loudest in the bathroom,becomes unbearable.last wednesday the whole system was powerflushed but the noise is still there.the plumber said its an old boiler and it will make lots of noise(suprima 50,installed since 1999,microbore).he changed the pump as he said mine was not pumping correctly,he also adjusted the burner pressure to 10.3 millibars as he said it was to low.the noise is far worse since he worked on the system.the only way i can get the noise to a bearable level is to turn the burner pressure down half a turn.im wondering if this is a sign of a blockage as the water in the heat exchanger is heating up but not circulating correctly.i also constantly have air in one radiator upstairs and from the system bleed valve next to the tank.ive removed the heat exchanger and flushed it out as best i could but it seemed quite clean after the power flush.can anyone suggest a permanent fix as im going mad.any help greatly appreciated.