Air in the rads is an indicator for bleeding - what sort of noise - is it air?
When you repressuise with the system cold then it should be between 1 and 1.5 bar. Your boiler MIs will tell you what it is for yours.
When the system heats up the water expands so the pressure rises. The expansion vessel absorbs the expansion and limits the pressure rise. It may be that the expansion vessel has lost some pressure or the internal diaphragm is damaged.
If the system pressure rises too much the PRV operates and discharges through the drain until the pressure returns to a normal level.
So you shouldn't worry about the presure rising under normal operation from 1 bar to 2 bar or even a bit higher.
Its the noise thing that concerns (interest not panic!!) me - can you describe it a bit better?
hi 1st thing if the pressure gets high only when the boiler is working ie fired up then the fault could be with the expansion vessel or it could be a partial blockage either way i would get it looked at just incase because its not working the way it should. 2nd thing with the rad need constant bleeding especially as often as you're sayin sounds like a small leak and air getting into the system thus causing noises. hope ive helped
Your expansion vessel needs topping up or replacing. As the gauge rises around to 2.5 bar with the heating on, check the pipe outside (copper), this usually goes straight down or curls back to the wall. It will more than likely be dripping (Pressure Release Pipe) if this pipe is dripping when cold or when the pressure is less than 2.5 bar the Pressure Release Valve is damaged too. This is where your loosing pressure. As the water content of the boiler drops, it will draw air in through the AAV (automatic airvent) and also if the boiler is lower than rads, they to will drop in water level. The vessel should be pumped up to 1 bar of pressure. and if re-occurs it will need replacing.