Gigantic fan. Listening to music or movies/TV on headphones without it seems almost painful now -- I sound like I'm exaggerating but I'm not.
Once you get used to audio always sounding like it's coming from outside of your head, going back to non-spatial audio that seems like it's emanating from inside of your head is jarring and positively claustrophobic.
And nobody's as surprised as me to find myself typing something like that. I always imagined it would just be a gimmick, but at least on the AirPods it's anything but. And it wasn't until the "Spatialize Stereo" toggle appeared recently that it worked with everthing, like regular tracks on Spotify.