Truck Forums - noise from dashboard

I am getting a humming like sound coming from underneath the dashboard of my 2004 F-250 SD. It kinda sounds like an annoying little blow-dryer. It comes and goes (mostly comes) Turning off the heat and air does not make any difference. If I turn the engine off and back on again it is gone for a short while but comes back.

Does anyone have an idea where this is coming from and how to quiet it? It is driving me nuts!!