No air or water - noisy radiator

[Ange Hook]

Hi, hoping for some advice with radiators that I don't seem to be able to bleed.
This is my second winter in a new build. The towel rail was making a knocking/pinging noise last winter so when I recently had my boiler serviced the engineer adjusted some screws on the rail itself and it's actually a lot better.. it just seems to ping now but the knocking has gone.
Two weeks later and there now seems to be a knocking noise coming from the pipes of my bedroom radiator (occasionally a rattle too) which definitely wasn't there before.
It's when it heats up so I'm guessing expansion noise but I find it odd that it wasn't doing it before so I thought maybe the radiators need bleeding, even though they all feel nice and hot.
When trying to bleed them I've found that several don't allow any air or water out at all. I don't think anything is blocked on the valve itself so just wondering if anyone has any ideas as to why i'm not able to bleed them?
The knocking noise is driving me mad, it wakes me up not long after the heating comes on in the morning so I'm desperate to try and figure out what's causing it now.
The pressure on the boiler is at 31 which I think is ok.

 

[Deleted member 120897]

What type of heating system? Eg combi boiler, tanks in loft etc.

 

[Ange Hook]

What type of heating system? Eg combi boiler, tanks in loft etc.

It's a newish vaillant combi boiler, no tank in loft.

 

[Deleted member 120897]

Pressure will be measured in bar, or on doted lines I think on that model. If it is indeed at 3.1 bar, then that’s too high. As for no water or air, then you may have a blockage, a kinked pipe or valves not turned on.

 

[Ange Hook]

Pressure will be measured in bar, or on doted lines I think on that model. If it is indeed at 3.1 bar, then that’s too high. As for no water or air, then you may have a blockage, a kinked pipe or valves not turned on.

Thanks for your advice, I'll have a look again at pressure and check the valves.