Worcester Bosch 35CDi II banging & wild pressure (CH only)?

Hi, just recently our Worcester Bosch 35CDI II boiler has been making worrying noises

After 30 seconds or so of the Central Heating coming on banging/groaning/chugging/train-like noises start and the normally steady grey pressure needle starts waving/fluctuating wildly just below the static red needle (1.5bars).

This only happens with the CH, NOT with HW.

I've googled but although there are posts about general pressure issues with this boiler there is nothing isolated to CH.

Every 2-3 months I bleed any air from the radiators and top up the boiler pressure but haven't done that for a while - I'll do that tomorrow but as the radiators are usually evenly heated I suspect it's not the problem.

It would be great if it was that simple but if not is it more likely to be a heat exchanger, expansion vessel or some other issue?

Appreciate any thoughts.

Cheers

gf