I have a lot of hummingbirds in my yard and recently I keep hearing a loud chirp.

Because there are other birds around I wasn't sure which bird was
making the sound.

It wasn't until I was able to stand quietly in the yard and watch a hummingbird swoop down by a female and then rapidly fly upwards
(in a large "U" pattern) and make that sound did I learn it was the
hummingbird mating call.

Below is what I learned -
http://www.cosmosmagazine.com/news/1829/hummingbird-sings-with-its-tail-feathers