This morning in particular I've noticed the vacuum hiss is constant when in the off position, but maybe it's more apparent without the noise from air flowing through the vents. I'll likely have some time to confirm it this weekend, but as the sound is literally right there at the dash my most likely suspect is that AC Vacuum Control Selector, that rotory switch where the vacuum lines connect. The question is, if this is the problem what replacement options are there? I've been looking but found nothing which surprises me given how this seems to be such a common problem. This is for a 1978 98 with a Tempmatic unit.

 

There are a few different designs across GM. For example: I have an 85 G body that wouldn't select vents. It turned out to be a failed plastic rivet that held the vacuum selector together I replaced it with a sheet metal screw, and it was good. It sounds like you're on the right track.

 

Was able to work on it last night, the plastic rivet along with a lot of dirt was definitely the problem. One of the prongs on the rivet was already snapped and it was very loose. With the help of a friend we followed the directions in the video exactly, and while i haven't driven it that much in the limited testing I did I noticed a huge improvement. Vent works again and DEF switches almost instantly instead of the minute or so it was taking before.