Hi, i have recently installed a new central heating system which consists of 6 rads and a vaillant ecotec pro 24 combi boiler. All seemed to be working ok for a week until a loud vibration sound was heard from the boiler. It seems to happen intermittently after the heating has been on for a couple of hours. I assumed that the vibration occurs when the TRVs are closing, so i adjusted the automatic bypass valve. I also adjusted the lockkshield valves on the rads but the vibration is still happening. I have called vaillant helpdesk and they said it could be dirt in the system. I have flushed system through but no joy. Any ideas thanks?