I have a new problem. For those of you who read about the fiasco I
created by adding a matching transformer to our video conferencing
audio path, I have put everything back the way it was. We have noticed
a very intermittent buzz that we think is caused by the
videoconferencing unit. However, this buzz gets MUCH louder when I
touch the SM58.

The connectivity looks like this:

Video unit to mixer via unbalanced two-wire connection;

Mixer to SM58 via XLR cable (two, actually).

What in the world would cause a buzzing in the system to get
significantly louder simply by a person touching the microphone shell?
This isn't a constant problem. Right now there is no buzzing and I
don't know how to make it buzz for troubleshooting purposes. But when
the buzzing is present, touching the mic makes it much worse.

Any ideas?

Thanks!
John