Artificial pulsating sound heard, no craft seen.

The following report came from two friends of mine, "George" and "Bob" (pseudonyms).
 "We were outside building a deck in the rain, not making any noise ourselves except for, occasionally, hammering.  There was heavy, complete dark cloud cover.  We heard a whirring kind of noise."  George: "I thought it sounded like air blowing through some kind of metal tube."  Bob: "I thought it sounded like highly crafted ball bearings rolling, pulsating.  Then it grew louder, we came out to the front of the yard to listen, and then it faded away pretty quickly."