2005 Pontiac Montana Hissing from dashboard

When I switch from my defroster to my air conditioning, the air conditioning does not work and there is a "hissing" noise coming from the dash/vent area. I am assuming that this is a vacuum line issue. Would my problem be in the passenger compartment behind the dash or elsewhere on the vehicle. Prior to this the a/c was working fine. After sitting for 2 weeks, the next time I used the vehicle, this problem was present.