My 8 year old Vokera now makes a high pitched chattering noise when the radiator thermostatic valves are all closing in. I have an always-open single radiator without control valves as a bypass; but the noise still sets in as the other radiators close off. Opening a single thermostatic valve reduces and stops the noise (and restarts it if I re-close it). The sound is coming from low down in the boiler, near to where the diverter valve is (from the diagram in the manual).
The last plumber said that I needed an extra bypass line. This hasn't been necessary for seven years so I suspect valve wear as the cause.
Any expert advice please before I have my constructive discussion with the plumber when he comes back.