I have a dental crown and when I sing a particular high note, the metal post in the crown vibrates and becomes so intensely uncomfortable that I have to stop singing. Please explain what's happening.
It's sympathetic vibration!
Most objects have a fundamental pitch. Flick a glass, and it rings at a certain frequency. Sing in a bathroom, and one pitch (a very low one) will sound louder and richer than others. Blow across the top of a bottle, you'll hear one note - its fundamental pitch.
Seems like your crown/post has that pitch on the note where it hurts. Your voice, mouth, tongue all vibrate at the frequency of the note you sing. When that matches that of your crown, it recognises it, and gets excited. And joins in!
There's not a lot you can do, except keep off that note. A note even a semitone higher or lower will be ignored, so you can choose a key that doesn't include it.
Could bring a whole new meaning to singing false-tto...