You have a vacuum leak in the vent system.

If it's been leaking long, the vacuum pump has probably burned up also.

Start by following the black vacuum line from the pump on the left shock
tower, to where it enters the cowl on the right side, by the heater valve.
There's a T that goes to a vacuum chamber in the cowl that often breaks.

Work your way through the dash, until you've eliminated all the leaks, then
replace the vacuum pump if it's not operating.