Clicking sound (relay ) from Baxi Solo 3 as it switches off?

Hi, My Baxi Solo 3 is working correctly until the CH/HW is switched off. the pump runs for another 10 mins as it should but instead of switching off, their is a regular clicking sound (relay) and the pump pulses with each click. The time between each click gets progressivly longer and its all quiet after about 1 minute. I am an electronic test engineer.