1995 Transport Van with 250,000 miles has a dull thumping sound after starting ignition and before shifting that seems to come from front passenger area. It will be 3 or more repeative dull thumps right away and not every start up time and only during that time.
Part of the HVAC system is malfunctioning. A blend door or another part that is moved by a servo motor is attempting to move, but it is unable to move, and that is the sound that you hear.
Even if you don’t have the HVAC system activated, many of these systems provide a minimal amount of air circulation at all times, thus you may hear a noise from the system even if it is not turned on.
The vent door actuator requires replacing. This system is designed so that when the doors are closed the vent is open so air can escape from the interior of the vehicle. When the ignition is turned on, the vent door is suppose to close. The actuator for your vent door is going past it’s travel causing the vent door to open and close repeatedly.
Tester
Are the air flow-control doors in this HVAC system vacuum or electrically-actuated? If the former, it could be just a vacuum leak. All the doors shift position when engine vacuum develops. OP, try parking it with the HVAC controls set in different positions and see if that makes any difference.