I thought it was normal in some children as well. My daughter does it, and my son just started (but I think he enjoys the noise it makes). One of our health care providers have said it is indicative of temperament, and a Chinese herbalist we see said it relates to the release of heat in the system, particularly in children who tend to run hot. (I *think* they mean heat in a metaphorical sense....)
I asked our ped dentist as well, and she said a lot of kids do it and outgrow it. I wouldn't stress!
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