a female Amethyst-throated Sunangel (Heliangelus amethysticollis) sleeps in Peru. READ THE WHOLE DESCRIPTION BELOW BEFORE ASKING QUESTIONS.
EDIT: Alright, Thank you Reddit for blowing this up. Lets clear a few things up here. The bird is in a container that is attached to machines that measure how much oxygen the bird is consuming. The noise you are hearing is the hum of the machines in the background (the main one being the FoxBox... The noise is actually a lot more quiet than it seems, for whatever reason my camera picked it up and made it sound a lot louder. This experiment was performed with the guidance and supervision of some of the top experts in tropical ornithology. The investigation was fully permitted and performed in a world renowned research facility. This bird was not harmed whatsoever, it was fed with sugar water throughout the experiment and was released safely. All of the hummingbirds measured like this consumed a very low amount of oxygen at a very stable level as compared to other, larger birds, which suggests that they were in torpor, or a state close to torpor. After the experiment was done, I watched the bird fly away myself, it was fine. And yes, I know it may not actually be snoring. Even the supervisors of the project are unsure why it is making that noise. I have asked them to look into it since this video has received so much attention. I have been studying ornithology for years and am currently a Masters student studying birds. The welfare of birds means the world to me, and I am dedicating my career to their conservation.
EDIT #2 (2-Mar-2012): It's likely that this bird is in the early stages of arousal from deep torpor after disturbance. The gaping of the bill might be a way to breath deeply and bring in plenty of oxygen. When they are disturbed in torpor, they try to warm up as quickly as possible and that involves intense shivering. But initially, they are too cold for high-speed muscle action so it's hard to see the shivering movements. The high pitched squeaking sound it is making is likely a cute side-effect of the gaping for oxygen.