I think I found out why the window is buzzing. It’s from the flashing on the edge of the roof! The window makes the noise, but it starts up on the flashing and it conducts down to the window.
I figured it out because it was finally warm enough and dry enough but windy as all hell, so while the wind was blowing and I sat there with my head out the window until I started hearing the buzzing noise. I’ll have to call the roofers tomorrow. It’s been a several years since we got the house re-roofed. I wonder why it started buzzing all of a sudden? Ah, well, at least the mystery is solved (or so I hope).
Like the lawyer told me, though, it didn’t make noise before the window replacement and it makes noise now, so I’m guessing the installers accidentally hit the flashing with their ladders. At least this is something that I might be able to fix myself.