I agree with Agile regarding gas pressures. A gas safe engineer will check both working and operating pressures for you. I also advise not altering the gas isolation valve but I'm pretty sure you only did that to test it? Sometimes air restrictions in the combustion chamber can cause howling as...
It sounds to me that your expansion vessel is flat mate. Air should come out the valve. If it's flat the expanding water in your system has nowhere to go so the pressure builds up and the pressure relief valve dumps it. The expansion vessel should be pumped up to around 1bar. The same for the...
If there are no visible signs of a leak anywhere apart from the filling loop as you turn it (this is common) and the pressure relief valve is discharging you will more than likely find you have a flat or burst expansion vessel in the boiler. This means that as the heating water gets hotter it...