zafira vacuum problems

i have a zafira 2ltr dti with another problem next to the battery there
are two small cylinder shaped objects on top of them it says vac out
they have electrical contacts to them , one of them is buzzing and when
you rev the car it buzzes louder also the car has started running
erractic as in if you hold the revs at 1200rpm the car starts missing
is this related to the vac out

all help appreciated

 

check the pipes leading to/from it are not persished, sounds like a leak
where air is getting in and a vacuum cannot be formed.

mike