Ok, this happens in Jabba's palace when Han gets thrown into the cell with Chewie. I can't remember the exact line it happens in (I think it's just before "Luke! Luke can't even take care of himself much less rescue anybody.") Han leans back and squints his eyes as if in surprise, kind of opens his mouth, and Chewie howls. It looks like Han's the one growling. Every time me and my girlfriend see this we crack up and keep rewinding it for a solid half hour.. Hehe. Good luck finding it.