Potterton Flow Return

I have just bought a property with a small Surprima boiler, (no hot water and it is in an upstairs cupboard) my problem is noise from the pipes, under the the floor boards, a real drag as it is boarded over the top, under the carpets! I guess the problem is just the pipes expanding and contracting, however, from the sounds of the water flowing as it starts up, it seems as if there is air in the system, for example in the towell rail in the bathroom, you hear the water fizz, for the first 5 - 10 minutes, as it warms up. (heat set at boiler is low and a gauge tells me it's only 50c. The bottom line is I don't want to take the floor up and the fault if in in fact air or a flow problem. Note the pump is on slow, the water makes to much noise if not, there is no cold spots on the rads.
Can people tell me? is it correct that the expansion pipe rises to the header tank from the flow output, the pipe comes out the top has a branch up to the header, then down to the pump below, also I note the cold feed joins in the same way on the return, the other side. Hope people can help. DJR
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