The vents were acting semi-normally. During steady cruise, or at idle, they would receive sufficient vacuum to do their thing, but when the throttle was opened, they would slowly reverse position (if engaged) accompanied by a hissing noise. This noise was readily apparent, and coming from somewhere in the vicinity of the vacuum tank, NOT from the failed check valve under the hood. Apart from the normal sounds of actuation when a button is pushed, all is quiet now.

I just got this car a week ago, and as I haven't owned a 240 in many years, I am not all that familiar with their peculiarities any more, so I didn't immediately notice that there was anything unusual going on with the air vents.