Yeah that "silent photo shoot" feeling is weird. What made it so quiet up there, @iris_hall? Like zero wind or something?

Fan blades hitting at specific frequencies can set up a harmonic in the spindle housing. Over time, this doesn't just affect bearings but can warp the housing itself, making true alignment impossible. I've heard of shops where they had to scrap entire spindles because the wear patterns were too complex to fix. It's one of those things that makes you paranoid about every little noise, lol.