I was going to say a belt tensioner or other belt bearing was failing, but it sounds like you've narrowed it down. Vacuum leaks can cause some pretty horrendous sounds. If you're pinching a hose running to the firewall, then heater controls or servos is more likely. A hand vacuum pump will allow you to check them all out and find the leak. As for the control, sometimes a regreasing of the vacuum diverter switch guts will solve those types of problems.