There is a great video on science.com showing how hummingbirds make the sounds they do when diving.

Find the video here: http://video.sciencemag.org/News
Explanation here: http://news.sciencemag.org/sciencenow/2011/09/dive-bombing-hummingbirds-let-th.html

Not sure if you will have to sign up for the free membership to view them or not.

Enjoy,
Ron McCluskey
rmcclskyatmindspringdaughtcom
Cheney,WA