my oldie sometimes does this. if i wrap her in a blanket she will stop in about 30mins or so. i dont know why she does it. it isnt because it is cold. she just seems to need comfort for no apparent reason
with Archie being so unwell i am not sure if it can be a sign of pain or anxiety. hopefully someone will be along soon with more info
Our greys do this sometimes. It worried me to death the first time, then I found it's quite common. Usually it happens when they are relaxed or dozing (so that's a lot of the time for a grund!), and they will chatter their teeth for a few minutes. It just seems to stop naturally, so now I don't worry about it.
Both mine do it - Nancy does it when she's doing something really exciting (so at the moment that's most things), the other guy tends to do it when he smells something awesome (so fox poo, rabbit poo, dog poo... spot the ongoing theme here?)
Our springer goes through phases of chattering her teeth for no apparent reason and it's always when she's just coming off a course of steroids (she has recurrent SRMA). I remember someone posting on a dog forum about their dog chattering and he too had started doing it shortly after a course of steroids. Not sure if that's relevant at all though.