Probably a vacuum leak is letting the vent solenoids relax, it's not the
end of the world, but it will probably get worse over time.

There a vacuum hose from the manifold to the firewall that supplies
vacuum to the reservoir that sits on the floor under the center console
to power the push button vent switches. The nipple at the manifold is
too big for the hose, so there is a reducer that uses a small piece of
pipe to make a 90 degree fitting. The vacuum hose that fits over the
pipe splits and drops vacuum to the reservoir. When the motor is shut
off, you hear the air rushing in to fill the vacuum in the reservoir.

Bob