I have frequently heard the tremelo call of a loon, as it flys over a
wilderness lake. The Common loon has three main calls: the tremelo,
the wail, the yodel, and a lesser sound- a hoot.
To specifically answer the question, I'll quote from my well used copy
of the book Loon Magic: "Loons often use the tremelo to communicate
while flying; in fact, the tremelo is the only call loons can give
while in flight."
This is the same sound the loon gives when an intruder arrives,
including fly over planes.
To most Barb, the tremelo sounds like a crazed laugh. As an avid canoe
and kayak camper on Adirondack wilderness lakes, my most favorite loon
sound is not this one, but the wail. It defines an evening on a wild
Adirondack lake. OK, time to go north!