I've read that its from being able to see prey and being frustrated because they can't get at it. Zissou does it also when she's outside on a leash since she can't get to it because I won't let her. When she tries to chase something she never does it.
On the other hand it sounds kind of like a bird, maybe its a bird call????
my new kitty just yowls for now, but our other kitty does it, we call it chirupping. it's precious, and yeah, it's normal. if you were your cat, wouldn't you do the same? try it...without sounding like a dolphin...go ahead...
Duke and Sibohan both do that it's entertaining... you kind of imagine them going "here bird, here bird just want to play with you bird, I'm you're friend"
All Sibohan wanted for chistmas was a bird but I had to disapoint her *smile*