The first time Steve heard it, he wasn't even sure what it was. Nobody else seemed to be picking up on the low, quiet, steady bass rumble, or if they were, they weren't bothered by it.
Perhaps it was something in the house. He hadn't had much time to familiarize himself with Stark -- now Avengers -- Mansion before Zemo was trying to blow up parts of it, but now that the hazard was past, the Avengers were gathered in the still-intact grand dining room, chomping away at a massive assortment of strange foods in small white cartons. (Tony said it was Chinese; most of the Chinese that Steve had encountered before the war ran steakhouses.)
He'd picked something that looked mostly recognizable -- meat and vegetables stirred up with noodles -- and he had to admit it was great. But it was one more strange thing, and so he'd been focused on that when he'd heard the hum, and hadn't noticed it at first.
No, not a hum, more like a purr; it had a sort of vibration to the noise. Perhaps Tony kept a cat. He seemed like the kind of fella who might even keep a big cat like a tiger, and let it prowl around at dinner. Or --
He cast a glance at Hulk, who had plucked an entire roast duck out of the mass of food and was systematically eating the whole thing, though Hawkeye -- Clint? -- was nipping little chunks of it when Hulk wasn't looking.
But as he listened, the sound seemed to be coming from the other side of the table, near to Tony and Jan, who were amusedly watching Thor try to make conversation with Ant-Man -- oh, what was his name, he'd have to ask someone -- who was carefully feeding sweet sauce and bits of broccoli to a small swarm of very well-behaved insects. Maybe it was the insects.
He cocked his head, turning his ear to get a better sense of the sound. Jan was talking, but Tony --
That was it -- Tony was humming, almost below the threshold of hearing now, a deep purr, not a song or a tuneless amble but a solid bass noise. Tony with his strange gold eyes and brain and mouth both going a mile a minute (though at the moment he wasn't speaking), and his startling kindness.
Well, perhaps there was something different about Tony, like the Hulk only less noticeable. None of them were ordinary, obviously. Jan shot lights from her fingers and Hawkeye had unusually good aim (and an extremely broken nose -- there was a story there, Steve was sure).
And anyway why should they be? If they were going to be a band of superheroes for this modern age, they might as well be strange ones. If Hulk was huge and green, and Jan shot stingers and Steve was a science experiment, there was no reason Tony couldn't purr contentedly at the dinner table. It was a nice sound, after all.
It got so that Steve started to be able, after a while, to gauge Tony's mood by the noise; a useful tool, really, in handling the leader of the Avengers -- a man who could also be, well, moody. Steve started learning little tricks that could start the purr up, like a well-timed meal, or the presence of at least three other Avengers. Whether the Avengers were even paying attention to Tony didn't seem material. If you got Tony and three other people from the team in one place, the purr would rumble to life, most times.
Steve found that T'Challa and Jan and himself were the most likely candidates to start it up, but he'd even purr around the Hulk, if Hulk was being entertaining enough. Tony just liked being around people, Steve thought, even if most of the time he retreated down to his workshop.
(Robots could also induce the purr.)
It was useful because Tony also tended to lie about whether he was fine, whether he was happy, and Steve could catch him in it often enough to make sure the other man didn't get...neglected, Steve supposed. He angled for the purr. Sometimes just a smile or a cheerful word from him would trigger it, which was sort of sad, that Tony was so unused to approval and affection.
"Do you, uh, do you hear Tony...humming sometimes?" he asked Jan once.
"Does he hum? I guess I didn't notice," she said. "What song?"
"Not songs, just, when he's happy he makes this...noise."
Jan's brows drew together. "Are you sure?"
"Yeah, I'm pretty sure," Steve said.
"Point it out next time -- I mean, obviously be subtle," she said. But when Steve got her eye and tipped his head at Tony next time he purred, over a team dinner with everyone present, she cocked her head to listen and then shook it.
"Must be super-hearing," she mouthed at him. Steve shrugged, even though the purr was so strong it almost felt like it was vibrating in his own chest, and passed Tony the salad bowl when he asked for it.
After the Skrull invasion, Tony didn't purr for a long time.
Steve worked hard for it, doing what he could, but he had other concerns as well -- his broken shield, the Avengers' soiled reputation, the public outcry against them. None of them were very happy, and while he tried Tony's favorite treats and made sure that at least part of each day was spent with the team, he missed the deep rumble in his bones.
Then came Loki and his little curse that reverted Steve to how he'd been before, and everything changed.
Steve had almost forgotten how much larger everything seemed when he hadn't had the serum. He'd forgotten the muffled way everyone spoke when he was half-deaf in one ear, and the way the world seemed to tip-tilt sometimes with his vertigo. The shield, normally a part of him, felt like an impossible weight on his arm. For the first time in their friendship he had to look up to see Tony's face.
And it wasn't exactly gratifying to have to be rescued by Clint, or to learn that the curse on him and Tony and Thor…was really all about Loki being angry with Thor.
"Well, it's never nice feeling like a bit player in your own life," Tony said, after Loki's plot was foiled and all of them had been restored.
"I'm just happy I can breathe again," Steve said. The revenge of his asthma had been less than fun.
"How are you feeling? Any side effects?" Tony asked, peering into his face. He poked Steve's bicep. "Muscles feeling okay?"
"What are you, my doc?" Steve asked, grinning. "Fighting fit, I promise. How about you? What's the square root of three thousand and sixty-two?"
"Really?" Tony rolled his copper eyes. "Fifty-five point three three and change."
"I'm sparing you the decimal places."
Steve laughed, and Tony grinned at him, nose still very close to his own, and then there was a rusty rumble as the purr sparked to life.
Steve knew his face must have lit up, and he couldn't help pulling Tony into a hug, arms around his shoulders, Tony's around his chest, just above his waist.
"Good to have you back, Iron Man," he said.
"I've been right here," Tony said, but it was only a brief interruption in the solid hum vibrating against Steve's chest.
"Yeah, I guess so," Steve agreed.
Tony tugged back a little, and Steve let him, but he didn't move away; just adjusted his stance slightly, face almost level with Steve's.
"Tell me if I've got this wrong," he said, and Steve found himself on the business end of his first kiss in seventy years or so.
He was too stunned to do much, but he managed to lean into the kiss and curl his hands against Tony's shoulders, which seemed like enough, given the purr was starting to resonate so hard it filled his ears. For just a second he felt genuine delight, pleasure and triumph all in one --
And then Clint's voice echoed down the hallway. "Hey! Come on, what's holding you guys up! Team debrief time!"
He pulled back before Clint stuck his head in the doorway.
"Stop wallowing in your triumphs, old man! If I have to do a debrief, so do you! Come on, Brainiac!"
Tony groaned in dismay, and Steve shook his head.
"Take a break from debrief, Clint, you've earned it," he said, in his best earnest commander of men voice. "We'll do it tomorrow. Get a hot meal and some rest."
"Don't have to tell me twice!" Clint said with a whoop, and vanished again down the hallway. "Hey Thor, Dad says we're off the hook! Let's grab Hulk and go for burgers!"
"No post-mission report?" Tony asked. "You sure you're back to normal?"
"I think I have something more important to hand," Steve said. Tony rumbled by way of reply. "Come on. Clint will have cleared everyone out the kitchen. We can have a nice meal together."
"And maybe some dancing?" Tony asked with a wide grin.
"Yes," Steve said, grinning back. The purr intensified, filling him with warmth. "Maybe some dancing."
He woke the next morning to discover that Tony purred in his sleep.
"Do you know that you do that?" he asked over breakfast, as Tony hummed his way through scrambled eggs (which he could just about manage cooking) and Steve supervised the bacon and the pancakes.
"Do what?" Tony asked.
"You hum. Very low register. When you're feeling particularly self-satisfied," Steve said.
"I do?" Tony asked, perplexed. "Am I doing it now?"
"Mmhm." Steve nodded. "I'm not sure anyone else can hear it, though."
"How long have I been doing it?"
"As long as I've known you. And since last night," Steve said. "You don't even snore, you just…purr."
"Like a cat? Steve, are you sure you don't need a hearing check?"
Steve wrapped an arm around his waist, kissed his hair, and let him go. "I'm sure."
"Well, let me know if it gets on your nerves," Tony said.
"Never has," Steve replied, flipping two pancakes onto a waiting plate. He handed it to Tony, who helped himself to some eggs and bacon and sat down at the kitchen island, close enough that a few seconds later, Steve anticipated, and then felt, Tony grab his butt.
"I really purr, huh?" Tony asked, laughing when Steve swatted him gently. "Wonder if it was all those genetic experiments Dad did."
"Howard experimented on you?" Steve demanded, turning to him, horrified. Tony cracked up laughing.
"No, no, I was kidding," he gasped through his laughter, and a loud purr. "Oh man, your face though."
"Practical joker," Steve grumbled, putting the stack of pancakes in the oven to warm for the others, and settling the last few on his own plate. He turned off the heat to the pans, covered them, and settled down next to Tony, close enough to bump him in the side with his elbow.
"So what do we tell the team?" Tony asked, around a mouthful of food.
"Nothing, for now," Steve said. The purr diminished, almost faded out. "You and I need some time to ourselves first, don't you think?" he added, and the noise renewed in vigor. "We should -- work out where we are. What this means."
"Makes sense. Like a science experiment," Tony said thoughtfully. "Test our hypothesis before we publish."
"You busy today, soldier?" Steve asked. "I thought I might take my fella to a movie if he wanted to go."
"Sounds like a good first proof of concept to me," Tony said.
"Fine, then. I've got sparring with Carol this morning, and I do have to at least get Clint's report out of him. Meet you back here for lunch? Then a matinee?"
"I've got a lunch meeting for SI," Tony said regretfully. "How about hot dogs at the matinee?"
"Sounds good to me," Steve said, and Tony leaned over to kiss him, the hum his whole world for a second before he slid down off the stool and put his plate in the sink. (Steve cheered internally. Teaching the Avengers to pick up after themselves had been a task.)
"See you after lunch," he said, sauntering off down the hallway, and Steve sat and basked in the fading sound of Tony's purr until Carol broke in on his peaceful contemplation.
"You look like someone hit you with a happy stick," she said, taking a pancake from the oven and eating it straight from her hand. "Hey, do you hear that? It's like a...a humming noise."
"Hm?" Steve raised his eyebrows innocently.
"Getting fainter -- there it goes. Weird. You did hear that, right?"
"Can't imagine what you're talking about," Steve said, beaming.
"Uh huh. Anyway, see you in the gym," Carol said, leaving the kitchen with a second pancake in her hand. Steve cleared away his plate, stretched happily, and wandered off to find out what movies were playing that afternoon.