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Well, I don't know if it's normal or not, but I can do this too. However in my case it is learned, as I used to do a lot of scuba diving and I am both a pilot, and fly commercially quite a bit. The only "side effect" is that, as you say, you find it easier to equalize to changes in ambient pressure. Where some divers have to hold their nose closed and blow, to equalize the water pressure on the way down, I often only have to wiggle my jaw.
So, of course, now you can understand the first half of my username. The second half hints as to where I dive, but that's another story.
There is also a possibility of TMJ issues. In my case, my ears would "crunch" every time I swallowed, because my teeth were not alligned properly. That has since, been fixed to the point that it doesn't bother me anymore.
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