Emission control Air pump?

[alpac]

Recently when starting the X every morning I was hearing a whistle. When I investigated this noise I found out that it was coming from the air pump illustrated on the diagram below. It looks like the air pump starts running right after I start the X and keeps running for a little while before stopping. I understand that this air pump is part of the emission control system but does anybody know when and how it is supposed to work? When does it start and when does it stop? Just wondering as I never had such a thing in any of my previous cars.