There is a vacuum check valve but it is for the brake booster not the blower motor. I attached the info for this. Also, I attached the info for the blower motor.
The blower motor is controlled by the HVAC control assembly with a control wire and it runs through a resistor.
If the blower fan is increasing with speed then maybe we have an air leak in the airflow door. I think the best thing would be to get a recording of what the blower is doing and what it sounds like as you pick up speed. Thanks
Images (Click to enlarge)
Tuesday, July 14th, 2020 AT 3:00 PM