Worcester boiler 28cdi banging noise.

[Sudds01]

When operating hot tap banging noise coming from boiler, it's an intermittent fault. Investigations to possible problem diverter valve check operation ok, plate to plate heat exchanger removed, cleaned & refit. Gas to water heat exchanger removed & refit checked out ok. Checked continuity of wiring to DWH & CH found to be ok, checked gas inlet pressure 21.5 m/bar to 22 m/bar. Burner pressure 15 m/bar should be 15.5 m/bar, tried to adjust unresponsive.
Followed domestic hot water procedure, mode switch minimum setting no response, no drop in pressure, maintained 15 m/bar with tap running. Boiler cycles CH off 2 min 25 secs on 41 secs.
Any ideas, any Worcester experts out there - any help appreciated?

 

[boilerdoktor]

whats the system water like??
pump sticking?

 

[Sudds01]

Water is clear throughout the system DHW & CH no contamination either side, heat exchanger intact.

 

[Sudds01]

Think'n back I remember the pump get'n excessively hot,so I switch off the boiler,then after afew minutes switched it back on,the pump ran and the tempreture came down to normal about 60c.