Lance and Pidge humming the theme music from Killbot Phantasm 1: Journey to the Depths of the Demonsphere because they finally got the Mercury Gameflux to work on the castleship and they’re so excited.
(Also, they’re each excited to learn that they are not the only person on the castleship who hums theme songs a lot, sometimes without noticing or knowing why they’re doing it, but they are, at the moment, more excited to finally play their video game.)
Lance and Pidge humming the Game of Thrones theme song when they feel like something epic is going on (also when they’re vaguely annoyed with each other for some reason but haven’t figured out how best to apologize yet, so they use humming the song as an ice-breaker, trying to get each other to join in on it).
Lance and Pidge humming the cave music from Super Mario Bros. when things are getting kinda scary and they’re trying to tell themselves to be brave like a Paladin is supposed to be (and then humming the above-ground theme song when things are calm and going well).
Lance and Pidge humming the Mission Impossible theme song when they feel like they’re being total badasses or they need a musical pep-talk (and sometimes when they’re sneaking around the castle, using, “it’s totally a training exercise” as an excuse to play hide and seek and see who can catch the other most off-guard).
Lance and Pidge humming different songs from the Legend of Zelda games without even noticing, at first, but then they do notice and get into a debate about which Hyrule Field theme is the best one (but they both agree that even trying to hum the Shadow Temple music is Off Limits).
(Look, there are some things that you just don’t do to your friends, and reminding them of the one dungeon from Ocarina of Time that has never stopped scaring the crap out of them? Is pretty high up the list.)
Lance and Pidge humming the Pink Panther theme song, then having to remind each other that you don’t actually get a bonus to stealth from doing so (and going, “oh, hey, look who’s talking” but it’s playful and neither one takes it too hard because they both know that they do this and don’t mind teasing each other about it).
Lance and Pidge being ADHD buds who ground themselves by humming theme songs from stuff they like, and bonding over it because friendship is great and so is making friends who actually understand your ADHD stuff and support you with yours.