Oil Central Heating System

Hi

When I put my oil boiler on there is a tapping noise coming from the pipework upstairs.

When I switch the pump off the tapping stops but when I switch it back on again the tapping starts again.

Anyone knows what this could be? Obviously it is to do with the water being Pumped around the pipes when heating is active.

Thanks

It’s the pipes expanding and contracting with temperature fluctuations.
They have been fitted without an expansion gap and could be loosened.