Why does my rat chatter his teeth?

This is called "bruxing". It's similar to a cat's purring. The emotional response is either excitement, contentment, nervousness, fear, or any other strong emotion, so you would have to observe your rat's body language to understand if the bruxing is pleasure or not.

You can "talk" back to your rat by also bruxing. Just keep your mouth closed and grind your molars together. In some cases this has pleased a rat so much that it goes on to eye-boggling.

Unlike other rodents who chew on hard materials to keep their teeth in shape, rats brux for the same purpose.

Eye-boggling is what happens when the rat really gets into bruxing, and this is definitely a sign of pleasure.
If it's a really nervous sounding one.. then it is upset or nervous..

if it's more of a content sounding chatter, it's happy.. (they have teeth that grow so fast that it is sort of habitual to grind them)

Check under Bruxing and Chattering.. (they have sound files as well)
they can do that because they are board or there teeth are to long
Maybe it is cold, or it is ill.
There could be somthing serious wrong with him and this is his way of showing you. Take him to the vet- schedule it as a check up and then when you're in the room w/ the vet, tell her that he is chattering his teeth a lot and you r wondering if he is sick or could be showing signs of an injury or something. Good luck with your Rat!
Hamsters, rats, mice, etc. They all do that.
my four do it all the time and they have happy and healthy :)
This is indeed Bruxing, you should chatter back! Its a great method of communication! I brux at my orphans when dropper feeding them and it helps them respond better.
I have read many aticles about this partly because my rat does it too. Usually it's beacause his/her teeth are too long and this is natures way of shorting them. It can also be that he/she is either annoyed or frustrated at something. Don't be worried at all, she/he might just want more attention or might be trying to shorten his/her teeth. The only way you can really know which one is to insert a chew bar or something like thatg into the cage and see if it stops. If it doesn't try and figure out what he/she is annoyed at.

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