It's the brake booster. You don't hear it when it's running because the engine drowns it out, and it's under vacuum...
If your brakes work like normal, you shouldn't be concerned.
91 318is - M50NV (gone)
02 WRB Subaru WRX EJ207 forged, 320 AWHP stock turbo.
85 - 325
02 325i Sedan