What to do about ventilation duct noise???

My office at work is plagued by white noise from the building's ventilation system. The ductwork is exposed and runs along the ceiling and wall. There is a large vent just above my desk, which drones on throughout the day, all year long. Noise, both turbulant and mechanical, emanates not just from the vent, but also from the entire length of ductwork.
This is a pity, as I spend a lot of time here and thus listen to lots of music. As a matter of fact, my job is also affected by the noise. I am a psychiatrist and often have trouble being heard over the subtle roar (I have a soft voice, "soothing", in the words of my patients).

Cheap fixes, like taping cork panels over most of the vent, help a little, but they inevitably fall off after a while. Plus, it is not a terribly professional look. I have no control of the heating or cooling in the building, and would not be able to make substantial changes to the existing ductwork without being arrested or at least chastised savagely.
Any thoughts from all you audio savants?