If you can hear a sucking sound then its almost a certainty that you have a leak in the air intake/vacuum lines somewhere.
With the engine running, If you get a can of carb cleaner and spray on hoses in the area where you think there might be leak you will hear the enginr revs change as it sucks in the carb cleaner and burns it.
Or you could find an easily removable hose that you can blow cigar smoke down to see where it leaks out of.
The is proper smoke machines for doing this and a local garage to you might have one if you don't want to start chuffing on cigars.
There may well be a common fault on the D2's in that area and I'm sure one of the many D2 chaps will be along with perhaps better advice.