Hvac replaced 4 times

I have had a problem with my 2002 Blazer. I am having hissing from the HVAC. I have it replaced and 2 weeks later it is back. The dealership said the vaccuum switch on top of the transfer case was leaking oil and was getting into the Hvac and breaking the switch. I had the Vac swith replaced 2 weeks ago, replaced the HVAC and the vac lines cleaned out. Last night the hissing came back. No one knows how to fix the problem. Any help would be much appreciated.