Baxi Solo 3 PFL pcb relay buzzing?

Hi, Have a Baxi Solo 3 PFL 3 years old, working fine but when the boiler shuts off 1 of the relays on the pcb buzzes. As soon as the boiler fires up it stops buzzing. Everything works fine and I know "if it ain't broke don't fix it" but it needs sorting. Any ideas? pcb fault or 1 of the control switches/thermostats?