That vaccuum-cleaner noise (97 C230)

[Will_w202]

What part makes that whirring whine at startup? Fuel pump? It kicks in soon after you start the car, and if you don't drive off, usually stops after 30 seconds or so. The reason I ask is I have an engine that drops revs when idling for a while after startup, and this morning when it did it, the vaccuum-cleaner noise kicked in very briefly after I had driven a mile or so and was stopped at a light, when the revs dropped. I'm trying to find a correlation.