Vacuum leak noise from behind dash panel

Dear All,
My W124 230E sedan has developed a vacuum leak noise from behind the instrument panel.
 
I'm pretty sure that it is a vacuum leak as it disappears when the throttle is opened and car accelereates, then reappears when throttle is closed.
 
Any clues?
 
Thanks

One of the vac lines for a vent may have come off.
ohlord