I recently had my Baxi Solo 3 serviced and it passed with no problems. However, in the last week I've noticed that the boiler is making a vibrating noise when off (not fired up). The heating / hot water goes off at around 9pm but the boiler makes a low vibrating noise constantly. This can last for a few hours. The system itself seems to be working fine, still getting hot water and full heat to all rads. It's just this constant vibration that is annoying the hell out of me. Any ideas?