I really appreciate the ideas guys, I did the following today:
1. Rearranged my desk and moved the Pan Am about 2 feet from my computer
2. Took extra care to route my USB cable away from other electronics and their power supplies
3. Switched USB ports on my computer
4. Added the tube covers
5. Unplugged the Passport from charging while in use (I feel like I had tried this earlier, so I'm not sure if this is actually helping)
Some combination of the above seems to have significantly reduced the problem, unfortunately I did all of the above at once so I can't say which of them helped the most. I think I may have heard the sound once today thus far (again with no music playing), but it was much lower in volume and pitch and also much shorter in duration. It is definitely a large improvement over last night, when it was much louder and occurred frequently, so I think I can live with things as they are now. All I can say is I've learned an important lesson: Never underestimate the impact of electronic interference from other devices.
Now time to relax and really enjoy the HD650's.