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Optimus had never struck Starscream as someone who would sing, even before he could say that he knew anything about the Prime outside of his politics and prowess in battle. He wasn’t an unhappy mech, as Starscream had found, and he wasn’t overly reserved. Shyness certainly wasn’t an issue for the leader of the free universe.
He was just remarkably parsimonious with his words. Singing seemed like a luxury of language that Optimus wouldn’t take to. It was something other mechs could lend their voices to but Optimus was always careful about what he said, how he said it, knowing that many mechs were hanging on each word.
It was only when they were alone and Starscream was half-asleep, curled up against the Prime’s chest, that the seeker learned that that assumption wasn’t completely accurate. In his barely-waking state, it took him a moment to recognize what he was hearing – the sounds of a human song, a Christmas carol, one of the urchins had called it. A song about the cold winter season that had taken claim of the continent they were on; this one, apparently religious, though Starscream couldn’t bother himself to remember the details of the human’s myths.
And there were no words to help prompt his memory; he felt more than heard as Optimus distractedly hummed the tune, nearly half-asleep himself. The sound was low, the Prime’s baritone causing the hum to be more of a rumble that sent gentle, warm vibrations scattering through Starscream’s energy field.
He distantly wondered if Optimus would mind, if he knew he was listening to him hum the song. He kept his optics close, allowing himself only a small smile as he pretended to be asleep still, fearing that the sound would stop if Optimus knew he could hear. It was comforting in a way he would never admit aloud, so he allowed himself the luxury of enjoying it while he could, something deep inside of him glad that Optimus allowed himself even this small amount of music.