haven't heard it.. but bear in mind that modern cars still use vacuum for a couple of tasks.

might have been the hvac vac tank purging (assuming the heater contols aren't all elctric).

bear in mind the fuel tank vapor recovery has a self testing pump that pressurizes the tank and vent system to 1 psi as a self check to make sure it doesn't leak. there is a small pump for this- it is part of the emissions system. recall all the check engine lights for lost or removed gas caps before they made the filler tube sealable?

maybe you have a mouse?

just thinking out loud.