Educate me on the vacuum system

[brookspw]

I have a 1994 C280. I tried the search tool but I need some education before I can use what it pulls up.

1. For the last month I've heard what I think is the vacuum pump cycling every 30 seconds or so.
2. Now it is cycling every 10 seconds or so.
3. I have no issues with anything vacuum related (locks, etc) except for controlling the flapper on the center vents for A/C.

Questions:
-- Is this coincidence or does it sound like my vac pump is cycling, I suppose because it is losing vacuum due to a leak somewhere?
-- Why is it affecting one thing but not another?
-- Is the vac pump somewhere in the vicinity of the back seat?
-- What do I do next?

Thanks mucho.