Microphones buzzing every time my presenters speak

I host webinars each week.  

Every time my presenters speak, there is a buzzing sound.  It does not occur unless they are speaking.  It is uniform across 10 presenters in various locations using various microphones.  We are not in the same locations.  For some, the buzzing is worse than others.  In general, they use USB headset mics, the same mic I use, and I do not have this issue.  Some use old-school Apple earbuds that plug into the computer.  I asked one presenter to use her computer audio and ditch the USB mic to see if that would help.  It was better, but there was still buzzing every time she opened her mouth.  

For the most part, they are presenting from an office location alone in a closed room with their laptop plugged into a docking station with two monitors.  The USB mics would also be plugged into the dock.  They are on wired private company internet.  

Any ideas would be greatly appreciated.  They don't seem to take it too seriously because they don't have to listen to it.  We use interpreters for nearly every webinar, and they are all going to quit if we can't figure this out.