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I believe you mean "BI-LEVEL" mode, which means it should blow air simultaneously out of the dashboard registers and the lower register. The Temperature of the air blowing out of both of them is regulated by the temperature preset.
Does the fan still blow when you move the switch to BI-LEVEL? If it does but the air comes out of the DEFROST registers, that means your diverter is not functioning correctly, most likely due to a vacuum leak under the dash. If the fan shuts off in BI-LEVEL mode but still operates in HEAT mode, your selector module is bad. If the fan shuts off in BI-LEVEL mode and in HEAT mode, either your engine coolant is not hot enough to provide heat (automatic function), or the temperature sensor for your climate control system has failed.
Question. Is the flapping at a very fast pace that would indicate the speed of a fan blade hitting something? If so, you may be on to something there such as a misplaced gasket or something has dropped/fallen into the blower box via a vent.
If it is not that speed then see the following..
Hope I can give you some direction. Speaking from a generic sense all ac/heaters these days have a door or doors in the blower box that control air flow from the main part of the box to different positions when you ask for vent, heater, mix, defrost etc..
Most these days are operated with a small electric servo/motor and unfortunately the mostly have plastic gears that wear and can "thump-thump" as the motor cycles trying to open or close the flapper door.
Try to change the air direction from Max AC to regular ac to heater, defroster, bi-level etc all with the AC switch in the on position if you have a separate on/off switch.
Listen to the sound and see if it comes and goes with a different location of the selector switch.
One last question. Does it happen at idle or driving down the road at any particular speed?
Lets start with that and see what happens. This is interesting.
Usually this is a problem with the blend doors under the dash. You either have a vac leak, a dead motor, or a faulty door. The control opens and closes the doors to direct air thru the heater core and evaporator core along with directing air thru the vents. When a problem occurs, the default position is the defrost vents to make sure the windshield remains clear. You can do an internet search for "repair durango blend door" for more info.
the AC system in your car has small vent lines under the dash look on passenger side usually access door there. look in and see if you can see one of those lines have come loose if so push back in place and it should fix your problem. Also look possibly something has gotten in your AC vent system and one of those vent lines are plugged
The 1992 was prior to Buick putting cabin air filters in so there is not a possibility it is clogged. So more than likely your blower motor is going bad or your fan control switch is bad. Does this problem occur with heating as well as AC? Does it change if you switch between blowing on floor, face, window(defrost) If it doesnt change based on location of blow and type of air temp then I would start by turning fan on high and placing head (ear ) against dash in center of console area. Have someone turn fan knob from low to high with engine OFF...After goign from low to high and back a few times you can see if the blower motor is responding to signal from switch. Listen for a whine, squeal, rattle, or humm. A whine or Humm would indicate a bad blower motor... Squeal or rattle a bad squirrel cage/fan blower blades. If you hear no noise and no perceivable change in blower speed/noise then its possible your switch is bad. Unfortunately the only way to definitely diagnose either is to pull the switch from dash and use multimeter to test for voltage change and for blower motor to pull dash. There is a digital control unit for your car as well. Although rare sometimes the wires on this can loosen or it can go bad. But since you didnt state teh fan wasnt working at all and you are getting cool air, that woudl be teh last thing to check
When you say tinker with the A/C. what did he do??? Just want to make sure he did not hot wire anything to activate the compressor at the max position. If the system is low on compressor oil the compressor will not active to keep it from getting damage or low on R143A. Most place now will recharge the A/C system for $50.00 since they know that you can by a recharge kit for around the same price, but you need make sure you don't over charge the system or damage will occur. If you not familiar with the recharge procedure, have it recharged by a shop and if it's full then the problem may be a pressure switch and that takes around 5 min to replace and i am sure the shop will let you know before they try to recharge the system by hooking up the gauges to read the pressure in the system. Good luck and keep me posted, be glad to help you get some cold air again.
Check under your hood for broken or loose vacuum lines. Also, the pot on the hood? i believe, on that one. There should be avacuum line running to it. Looks like a little bowling ball. With your engine running , listen for a hissing.