1993 Plymouth Voyager Heater/ AC vents Move

On my 1993 Grand Voyager the heating and ac vents move from the vent position to the defroster position when you are going up a grade in overdrive. They will return if you let off the gas or downshift. I also have a 1992 that does this and a 1990 that does not. The annoying part is the vents make a high pitched whine when they return to the proper position. I realize the 1990 has a different heater control and air package unit then the 1992 and '93. They have done this since I purchased them (used and much less mileage). The dealer tells me it could be by design (I find that hard to believe) or it could be a bad diverter valve in the vacuum system. I have put heater cores in all of these vehicle and gone over every hose and fitting for leaks that I can find. I am not really worried about the '92 as it is entering retirement. The noise in the '93 is getting on my nerves. The '93 has a 3.3 liter in it and runs fantastic. It also has antilock brakes and does not use a brake vacuum booster so if it has a diverter valve I don't know where they hid it and my parts dealer cannot find a listing for one either. Is there any solution to this or have you heard of this before? Thanks for any help you can give, Don