i have a job that customer is complaining about sound coming from rads, its like a circulating sound, system is around 20 yrs old , but boiler is about 2 , (worcester combi gas) boiler enginner has changed the pump, sound still there, when you close th trvs right down the sound disappears, or when yu open hot tap, but i gues this is because diverter valve is now open to hot water right???? , does anybody know what it is??? im not gsr so canot do nothing to the boiler, but do you reckin its something else??? the trv's or something???