99% sure that its the expansion vessel.
You can check it by doing the following:
1. Isolate your boiler using the 2 taps which look like washing machine isolators on the 22mm pipes next to each other on the RH side of the underside of the boiler.
2. Drain the boiler from the PRV (bottom of boiler, red knob with "3 Bar" written on it. Turn it anticlockwide about 180 degrees until you hear water rushing out of the PRV pipe to outside. Leave the PRV in the open position, can be tricky but you have to do this!
3. With the PRV open to the atmosphere, and the boiler isolated from the CH system, get a car foot pump with a pressure gauge (£10 from Halfords it you don't have one), and climb on the nearest worktop to the boiler.
4. Locate the expansion vessel. Its a big red/orange thing on the back of the boiler at the top. It has a schraeder valve, like a car tyre on the top.
5. Put your foot pump on the valve, read the pressure. If its '0', then the vessel is flat. If its flat, try pressing the schraeder valve with your finger or a pen or something, if you get water out its a bad sign and the vessel will need replaced - not a DIY job!
6. If no water comes out of the valve, then put your pump back on the valve and top it up to about 0.8 or 0.9 bar.
If done correctly, this will take up the extra pressure created when the system water expands in volume due to being heated by the boiler.
After the above steps, remove your foot pump, shut the PRV, open the isolation valves, top the boiler up and run it on CH. Let us know what happens.
