Intermittent banging noise from boiler

details :
1) Potterton Profile ( about 3 or 4 years old )
2) No frost thermostat fitted – boiler located indoors
3) 3 speed pump – changes tone when the 3 position switch is operated but not sure how to thoroughly test to ensure it is actually pumping.
4) Banging noise happens when the timer switch is on or off
5) I disconnected the timer switch completely whilst it was making the noise the last time and it continued to operate.
6) Boiler constantly fires up / shuts down during fault ( every 30 seconds or so)
7) When I switch the boiler back on after a while ( a day or 2) it operates fine for some time ( a few days) before the fault reoccurs
8.) System does have a history of requiring bleeding via the radiators on a regular basis
9) System has independent Hot Water / Central heating ( it’s got 2 valves)
10) Fault started during a period when Central Heating not being used – only hot water ( still only using hot water)

Any suggestions gratefully received .

The switch on one of the two valves is sticking and holding the boiler on. It's banging because the valve isn't actually open.
As it's intermittent it's probably the HW one.
You can change the switch if you don't mind a fiddle and you're tight, most mortals change the head.

Sounds like you don't have a boiler bypass for the water to flow through. What size Profile?

That's great - thanks. The boiler is a profile 60. Funny you mention the bypass. Spotted a valve with no tap head on it and discovered it is shut - I am sure it's some sort of bypass. I've opened it now and the rads are hot so I've shut it again. No automatic bypass then. Next time fault occurs I'll change the HW valve

Open the valve a quarter turn!

Tony

Thanks Tony - presumably to prevent it seizing? Will do

No, not to prevent it seizing, but to give a very slight bypass!