a silly thing that I do
I like to drum along with my fingers when listening to music. But not in a "just tapping to the beat" kind of way. Instead, I pretend to play some imaginary instrument, probably one with keys ... so something like a piano or an accordion, depending on the orientation. There are no hard rules, but over the time, some conventions have emerged. For example, for chord progressions, I tap two fingers at the same time for each one, but leave out one in the middle, so "thumb-middle - index-ring - middle-pinky" and so on. For quick melodies, I do the same alternating pattern, but with only one finger at a time. Single-finger tapping in the regular order is for scale-like movements etc.
This habit goes way back - it started as a finger exercise when I was still playing an instrument. Eventually, I stopped doing that, and it instead became a way to "engage" with music when I can't whistle, sing or dance along. I only do these things at home, when I'm by myself. But sometimes, when I'm out and about, I still want to respond to the music in some way. The tapping is an easy, inconspicuous way to do it (and also easy to ignore, since I'm not using my nails to tap). I tell myself that by making it look like I'm "air practicing" some kind of instrument, it's less likely to be misinterpreted as me just being restless or irritated. It's a silly thing to do, but it doesn't hurt anybody, so I'll probably keep doing it.