Update..... I have disconnected the grey vacuum pipe from the engine bay to the interior of the car, as other people have said it runs from the central rear of the engine under the battery tray and through a rubber grommet in the bulkhead (firewall) to the vacuum reservoir behind the glovebox (RHD car)

After doing this the noise is still apparent and obviously not vacuum leak related as suggested by other posts I've looked at although they might make a similar noise.
I have noticed on this forum that another member has the same noise on their car, described at flutey, we have both recently had the propshaft central bearing replaced just before the noise started, I have contacted the parts suppliers and they've had complaints about the same thing so it looks like a faulty bearing is to blame. A new bearing is on the way and I'll confirm if this cures the problem when it has been fitted.