OK, what's the black vacuum part in the right of the attached picture, above the windshield washer reservoir and to the right of what I guess is an anti-lock brake controller?

When I park the car, this particular part makes a noticeable fast ticking sound, which tapers off and then stops after a minute or two. Obviously the part has something to do with vacuum, but I have no idea what. The car is mostly behaving fine, though sometimes when I open one door after parking and killing the engine, the remainder of doors do not automatically unlock. If I hit the button on the console, they work without fail.

Just curious what this part is and whether its sticking is normal. The number on the cap is marked "A0005450427." If replacement is necessary, is it expensive/difficult?

Thanks for your expert tips --

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI