The "dive noise" as it is referred to in the literature is one
of the many, as yet unsolved, mysteries of the hummingbird world.

My assumption when I first hear it was that it was a non-vocal
noise, like the zubbing in the Rufous Hummingbird dive. But the
jury is apparently still out on the subject....

According the BNA account:
"The Dive Noise ( Fig. 2C) is produced by males at the bottom of
their dive in a dynamic display; there is spectrographic evidence
that the sound is ?mostly, if not entirely, vocal in origin?
(Baptista and Matsui 1979). Stiles (1982) disagreed with the
interpretation, stating the noise is too loud in relation to the
size of the syrinx and air-sac system to be vocal."

Baptista and Matsui
http://elibrary.unm.edu/sora/Condor/files/issues/v081n01/p0087-p0089.pdf

Stiles
http://elibrary.unm.edu/sora/Condor/files/issues/v084n02/p0208-p0225.pdf

--
Mike Patterson
Astoria, OR
celata at pacifier.com

SNOWY OWLS on the Lower Columbia - links to current information
on the 2005 irruption event:
http://home.pacifier.com/~neawanna/SNOW/SNOW20051120.html