Oh I love the t-rex noise. Chickens are the closest living relative to the t-rex, after all. Mine seem to do it when something isn't right, either a hawk flies over, or I relocate a nesting hen, or get to close to something I shouldn't. I would have to agree with it being a warning call or defensive sound.
It's a sound that some kinda danger is around. Mine will make that noise if they see a shadow fly over the coop and will stand real still and make that noise. My best friend that works at a vet has some Emu's and when a large plane fly's over the male starts flopping around and kicking around like crazy It's sooo funnye He's trying to protect the eggs,but we have had lots of fun over there. It reminds me of a huge killdeer. Most of all I love the huge eggs and the color is so pretty oh and the babies