Even though this hawk appears to be vocalizing it's displeasure with me, I believe it's actually trying to disgorge a hawk pellet. Hawk pellets are bundles of indigestible items such as bones, teeth, feathers and fur that are spit out in a solid form. Since raptors and other birds of prey eat live food, and do not have teeth, they can't chew their food. So they use their beaks to rip the food apart and then swallow large chunks. Frequently, depending on the size of their prey, hawks and owls swallow their food whole. As the meal is digested the soft, digestible materials (such as meat) is separated from the hard undigestible material (such as bones, feathers, fur, etc.) in the stomach. When digestion is complete those indigestible items are spit out in a solid form called a pellet or casting. The hawk's eyes are temporarily opaque because their *nictitating membranes have moved to cover and protect the eyes while the hawk is trying to expel the pellet.

* http://en.wikipedia.org/wiki/Nictitating_membrane

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