benl1981 is the noise you're talking about the one that sounds like something's bashing from side-to-side inside the central vents? I get the same problem, not very often, but it seems to go away with a small hit to the top of the vents or by turning the climate control off, and then back on again. So I'm not so sure its an adjuster... it's more like something's flapping around inside. I haven't had a chance to pull anything apart, but I'm thinking the actual vent connections themselves might be the answer (the tubing).
Sorry, speculations are all I have at the moment