There is some sort of high-frequency sound that causes slight ear pain located in the subway tunnel between Porter Square and Harvard Square, maybe about twenty seconds south of Porter.  I noticed this today going outbound at about 4:00, and both inbound and outbound on a trip several days ago.  I have taken the T hundreds of times in my life and never experienced anything like this sensation. It is not any audible screeching of the train that causes the discomfort, like a slight popping of the ears, but some sort of high-frequency sound.  Interestingly, both times, it has also caused me to experience a little popping sound in my head reminiscent of those exploding nerd-like candies (I forget what they’re called) whenever I turned my head more than about 90 degrees or so to either side.