Your recirculator won't come on in certain modes of air direction. I can't remember all of them off the top of my head, but I believe it won't come on if you have the defroster on. Screw with it. As far as a sucking sound, pop the hood, and open your door. You need a quiet place for this, so if you can do it in the garage (with the door open, Dr. Kevorkian) that's the best. Start your truck, let it idle for a couple seconds, then turn it off. Roughly a quarter second after you turn it off, you should hear your intake take a breath as the vacuum dissipates. If it's the same sound you're hearing from behind the dash, then you know you have a vacuum problem. That's the poor man's way of doing it.
You can also put a vacuum guage on the intake manifold, and if the needle bounces, you know you've got a problem.
The HVAC control panel is operated by a bunch of wires as aforementioned. There aren't any vacuum lines behind the radio area that I'm aware of. Hope that helps!!