Tapping noise seems to come from beneath floor boards in 1st floor and amplified through radiators in bedrooms 1 & 2 only. Noise starts about 10 minuets after the combi boiler fires up in the morning and is intermittent with each tap gradually getting louder, then slowing and then stops for about 10 minuets before starting again. Could it be a pipe touching a joist? Have closed thermostat vales with no effect, so I don't think its them. Is it a case of lifting the floor boards to find out or do I need to call a heating engineer.