This system was fitted 18 years ago and is serviced every year. The boiler is a Stelrad, with a DAB 65 pump running 5 x 48" singles,2 x 24" singles and 1 48" double radiators. Recently the boiler started ketteling and making a loud banging noise when the boiler is working. I have flushed the system and added descaler repeatedly, 3 times over three weeks. Then the pump was relaced with a Wilo gold RS 60 , this had no effect except the noise was worse so the DAB 65 was refitted. The noise persists and I am at the end of my tether. Any idea as to what the cause could be? Do I need to think about a new heat exchanger or is there a thermostat in the boiler that may be at fault? Please help. ![Question :?: :?:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)