You sure the boiler is a Worcester 24i ? The single light on the front is red.
If it's turned off at the plug the whole boiler is turned off. If the noise is still there its water borne.
Since the pressure gauge is not on 3 Bar the filling loop would not appear to have been left open.
Have you checked flush valves or float valves in the WC cisterns are not making the noise which is reverberating back down the pipes. Are you on a shared main? ....does it pass through the boiler compartment?
Have you tried turning off the water at the stopcock to see if the noise ceases.
If it's turned off at the plug the whole boiler is turned off. If the noise is still there its water borne.
Since the pressure gauge is not on 3 Bar the filling loop would not appear to have been left open.
Have you checked flush valves or float valves in the WC cisterns are not making the noise which is reverberating back down the pipes. Are you on a shared main? ....does it pass through the boiler compartment?
Have you tried turning off the water at the stopcock to see if the noise ceases.