Heat only boiler banging in pipework

[DOUBLEPLUG]

Folks, system is an Ideal Icos heat only boiler topping up a thermal store upstairs so pretty simple flow and return to boiler. All is working fine other than an occasional bang close to the boiler that is strong enough to shudder back to the 210lt tank and make it shake also. Boiler doesn't sound as if its kettling or anything and there is an air admittance valve on the return which is working fine. This bang maybe happens about half a dozen times a day. Also pretty sure its not pipe expansion or anything like that. Any thoughts would be appreciated....

 

[muggles]

Could it be a zone valve closing? Does the noise reduce if you turn your pump speed down?

 

[DOUBLEPLUG]

No zone valve on the boiler loop. Not needed. Boiler just heats up the vented thermal store. Stat on tank needs to be at 75 and boiler on max approx 80 so tank stat controls. Pump is wiko with three settings and on 3. I did move down to 2 but no joy...