I have a old Trianco

Question

i have a old Trianco gas Boiler, that is still in good condition.when the boiler is satisfied on temperature and turns off, i have a rumbling noise.this i have been told is due to the pump shutting off the same time as the boiler.And to rectify this the pump should have a overrun stat,to allow it to run on a few seconds to pump the hot water from the heat exchanger.Is this correct and can it be done