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I'm not sure, but I have this problem too. It doesn't always squeak. I've noticed that it squeaks more when I press it off center. Try being really anal with your thumb placement and find out if it still happens. I noticed that my squeak really faded after I made sure to press it in the center more.
 
Yeah mine only does it when I'm off center when i press it.
 
Yeah mine only does it when I'm off center when i press it.

See, I think that means one of the sides must be under strain when you press more on one side than the other. Be mindful of it, and if it flares up, then maybe take it to an Apple Store and have them tell you why it's doing it.
 
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