I had the same problem, although not related to a conversion to R134. It was a partially stuck air door inside the heater box (plenum or what ever). The foam from one of the doors had come loose and was preventing the door from closing properly. Also varied with pressure inside the cabin. (windows up and down, more noise when down) What I did was let the engine idle (high mainfold vacuum), turn everything on and cycle all of the switches. Heat, AC the whole nine yards. Finally blew a chunk of this foamy stuff into the back of the center outlet. Problem solved. If this doesn't work, then you may have to get inside the heater box. Big pain in the a--. Hope for the best. Good luck..
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.