Sounds of running water?

possibly air in heating system are rads cold at top in places - bleed radiators top up system again if required i.e. closed.
possibly open vent heating system running over into tank, look in loft when happening is water running into small tank? if so check pump.
possibly is leak in other system turn of mains and check when house is quiet and tank not filling does water sound stop and tank level stay the same? are you on a meter does meter turn when all taps off and tank not filling? try these as diagnostic steps

Kenny F
January 2009