Like cats' purring. Because it usually occurs when petting, it may just be a function of pulling the ears back. Or it may be a verbal language cue. Their hearing is probably on a slightly higher pitch. If you speak in a low range, your rabbit may not even hear you.
Vision is focused on movement.
Move slowly and approach from the side if you don't want to alarm your bunny. Jumping up in the air and kicking one's heels. Happy to see you. Be very glad you have such a happy bunny. Treat yourself to some chocolate ice cream.