Bryan,If you have a drop ceiling, the FIRST thing I'd do is make sure that over your head and around the perimeter are stuffed to the hard surface above with insulation before you do anything eles.
Believe me I have considered stuffing the joist with fluffy insulation, covering with decorative cloth and forgetting about the dropped ceiling. It could happen!
Right now the ringing seems to be getting worse, maybe something is coming loose? This helps convince me the major problem is the duct work.