Got a little more info. Turns out it was wasps. Here is the updated
comment/question:

   I'm asking because of an odd nest of wasps that i ran across that
purred. I talked to several entomologists about it, but they
   said hives did not make any such sound. I played some recordings of
the wasps over the phone to them, but I think they
   thought I was putting them on. One beekeeper, a friend of a friend,
came by to hear them, and noted several other odd
   things about the hive's activity. She had never heard of a hive
making such sounds. Let me know if you run across anybody
   familiar with the phenomena.

Bill Truesdell
Bath, ME