expansion vessels frequently become depressurised when the system goes to over pressure and vents out through the overpressure relief valve. This valve then can remain in the open position due to sludge and magnetite /limescale particles lifting the seat. The diaphragm in the expansion vessel can become pin holed, and a clear indication of this is when you try to repressurise it water comes out of the repressurisation valve (looks like a car tyre pressure valve). At that point you have to change the whle vessel. These are syptoms and you need to find the cause...what caused the overpressure... why is the system loosing pressure? Expansion vessels can become depressurised through other leaks in the system and boiler as well. Clogged boiler return filters is another favorite for system failure... answer get your appliances serviced! :twisted:
The whistle coule be a few things, could be the diverter valve, a build up of limescale in the plate heat-ex or could be the boiler case has not been fitted correctly - but would normally also make the noise with the central heating on. Best to get it checked out.
Repairing the leak to the bathroom radiator should sort the problems with the pressure as long as there's no other undetected leaks. The leak will stop when the heating is on as the heat causes the parts to expand.
Don't think it's limescale build-up as we live in a soft water area. Could it be too high/low water pressure? There is a lot of knocking in the pipes which indicates high pressure but would the whistling mean air is getting in? The pipes to the boiler come from above itself even though it's in the kitchen cupboard!