I wonder if you could get subjects who travel a lot to record, before going to sleep each night, in a secret journal they do not share with each other, about whether or not they experience a hum in any given location. If those journals substantially agreed, you might be able to get a clearer picture of what, if anything, is going on.

If there is any sort of community built around a potentially hard to observe phenomenon, it's likely going to be a mess sorting out true stuff from false stuff.