Clicking/popping sound from radiator pipes

[mrstiff]

We've just had a new boiler, hot water tank and 5 new radiators put in.
After approx 2 weeks we've noticed that our bedroom and lounge radiator (2 of the new ones) have started making clicking/popping noises.
It isn't as the heating has just come on or gone off and isn't happening regularly.
Do you know what it could be?
Is it the pipes heating/cooling?

[DEEARR2]

If there are thermostatic radiator valves fitted the, direction of the water flow can influence the valves. Should the flow direction not suit the valve install then the valves may click (or as you say pop) when the thermostatic valve head attmpts to close the valve. Worth having this checked.