They are eating Live Oak leaves. I noticed that clicking noise a couple of years ago when I had a few of these. If I remember right, it was a real fast clicking. They could speed it up or slow it down, sounded like a Geiger counter. It depended on how irritated they were getting. I never had one puke on me though, a lot of other caterpillars have but not this sp yet. I guess I've never irritated these enough for that to happen.I work in a lab that studies these guys it seems they employ acoustic aposematism by clicking their mandibles together before puking out a distasteful substance essentially their bile and last meal. the hairs on them are inocuous. Manduca caterpillars also click and puke. One of my lab mates was just published in Nature on the subject.