On July 5, we were hiking the Acadia Mountain Trail at Acadia NP. Near the upper sections we began to hear a distant "tseee-bzzz" double note. About ten minutes later we had flight notes of 2 (?) Red Crossbills near us, but unseen. Another 15-20 minutes later, the double-buzz started up again and I chased it down to get the recording here (apologies for wind noise). This is an outatke from a longer recording.
I never got to see the sound-source and it seemed to move from one tree to another. At one point I thought there might be two of them counter-singing. We've considered the possibility that this is not-avian: amphibian or even insect, and that could be. But the behavior of moving around sure seemed bird-like to me. Habitat was short firs and pines.
Suggestions most welcome!