T43 headset voice hum with analog or USB

[kmshuler]

When using Skype or Dragon Naturally Speaking, my headset microphones pick up a hum at about 60 Hz. The weird things are:

1. The when I speak into the Thinkpad internal mic above the Esc key there is no hum

2. The hum occurs whether I'm plugged into AC power or running on battery.

3. The hum occurs whether I'm docked in the docking station or not.

4. The hum occurs when I use my Plantronics USB headset (has its own internal sound card) or my analog headsets (which use the Thinkpad's internal sound chip)

So, my question is: Has anyone else experienced this?

Thanks very much in advance!

Kurt