Thanks Zip, I've had another look behind the glovebox from underneath, I can see the reservoir box like you said,
I haven't tried to remove it because I'm not convinced this is where the noise is coming from. The noise seems to be from the drivers side. These boxes are listed for sale as central locking vacuum reservoirs, but I've read that they are actually for the recirculation flap in the HVAC system, do you know which system it's for?
I've also read that the grey vacuum pipe that goes from the engine through the bulkhead/firewall does connect to this reservoir box and it can be blanked off if there is a leak. When the pipe is blanked off the grey box stops working and the recirculation flap will default to fresh air setting, I might try this to eliminate the box from my inquiries if it's true. The noise is particularly loud when the engine is producing more vacuum at about 30 MPH and when the car has been stood for a while suggesting that all the stored vacuum has been lost, the central locking is working properly even when the car has been stood for days which leads me to believe this leak and is nothing to do with the CDL system.