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Any ideas why the noise greatly increase when the system switches off, during normal use the systems seems fine. I understand the concept of pump overrun to cool the boiler down when the system shuts down but am totally baffled by the droning noise when it switches off.

For the last month whenever the CH system switches off either via the the room thermostat or via the CH timer I get a loud droning noise which continues for around 3-5 minutes and then ceases. It seems to be eminating from the loft where my pump and motorised valves are but I haven't really got...