I can't very well search this cause it's a broad subject, and I have no idea what could be causing this. I'm relatively sure this is the right place for the thread though.
Whenever I turn a vent element on, AC, Heater, vent, whatever, I get a vibration throughout the whole car. The first time I felt it I thought it was coming from the back end somewhere, under the car. Now, I can't exactly tell where it's coming from, but I know I can feel it in the seat rather than anywhere on the dash. It seems to mostly happen severely with AC on while coasting or slowing to a stop, and there only seems to be a small hint of it when the heater is on or at idle. The car only does this with AC or heater (any of that air stuff) turned on. I may be wrong about the vibration with the heater on though, I wasn't paying too much attention today. Was focusing too much on the excitement of being able to drive it after a long period of garage time. But I'm sure it happens with AC. It's way too hard to ignore. Any ideas?

 

Sounds like you have a vaccum leak. One of those old vacuum lines running around the heater controls may have gone bad. Or the switch itself might have gone bad.

 

I didn't think there were vacuum lines anywhere near the heater controls. I can look, I guess. Which switch are you referring to? I don't know how the heater switch itself could cause vibration. I realized last night I can feel it in the steering wheel, so I might have been wrong about it being under the car, per se. But I still don't know what could cause it.

 

The doors that divert air are controlled by vacuum. The vacuum lines run all the way to the controlls and then to the doors.

 

The vibration could be caused by your compressor locking up. That means it is going to fail soon. That is exacly what mne did. Just my 2 cents.