i have very loud banging from my heating system 5mins after it comes on.this is what is happening when the heating comes on its quiet but after about 5mins the system seems to stop then a series of very loud bangs from the pipes around the pump+3way valve area and a swoosh and it starts up again ok, ive been into the attic to the water tank and at the point of the swooshing sound there seems to be either air or hot water coming back to the tank?? ive bled the rads over+over again+bled from the 2 pipes with blanking screws above the pump in the airing cupboard no air. the boiler is a thorn emi apollo,switchmaster3 way valve+a grundfos pump, the pump was changed 2yrs ago but system worked fine up until now. it has been suggested to me that the 3way valve is not working properly causing a build up of pressure+heat don't know if this is a possibility?? any suggestions