AN: Oh my God, guys, so fluffy! Sorry, I've tried to write more for this story, but I have two papers due this week and only one of them is actually finished at the moment :S oops. Also I'm sort of freaking out with excitement for the season premier tomorrow, which is sort of short-circuiting my brain.

Diapason: (n.) A full, rich outpouring of melodious sound.

Nathan was a whistler.

It wasn't something Audrey would've ever expected when she first met the stoic and silent detective. In fact it fell into the category of things that she would never expect him to do under any circumstance. Even two years later, when she had gotten to know him better and to understand the softer side beneath his tough-guy facade, it still came as a shock to her.

The first time it happened, she thought it must have been a fluke. She wandered into his office at the end of the day to find him scouring through files and whistling to himself under his breath. For a moment she had only been able to stare at him in confusion. Then she cleared her throat and said, "Did someone put a little extra sugar in your coffee this morning, Sunshine?" Nathan had given her a sarcastic smile, the tips of his ears slightly red, and then changed the topic.

After that she started to catch him at it on random occasions. Sometimes it was his office when he was wrapped up in paperwork. Other times it was in the morning when he was waiting in his truck to pick her up for their shifts or to go to crime scenes. The few times she dropped by his house, she usually heard him doing the same. He always stopped the moment he noticed her, and any time she asked him about it he diverted, but she felt like she was on the verge of uncovering some new hidden secret about her partner.

As time went on, she started to hear it more often. He really was rather good at it as well; he never whistled those silly childish tunes that she knew, but complex, soothing melodies that she couldn't place but that she liked the sound of. When they had been working together for more than two years, she sometimes heard him whistling quietly when they were collaborating over paperwork. They caught each other's eyes and he smiled, but didn't stop. She thought that must be a step forward.

He was whistling when he came to pick her up for their first date as well – their first proper date, since their original first date had gotten a bit interrupted by her being kidnapped and all. The longer they dated, the more she heard him whistling. He would be whistling at his house when he let her in to have dinner together, or when they were relaxing together – or more often working on paperwork together – in front of the fire in his living room.

They had been together for about six months when he finally gave her an answer to why he whistled. "It helps me clear my head," he admitted over a homemade dinner at his house – he turned out to be a spectacular cook and they were Audrey's favourite meals. "It's something my mom taught me when I was little." Audrey suddenly understood why he had been so reluctant to talk about it; his mother was a soft topic that he rarely brought up even in passing. "She used to whistle the songs to me all the time. When I got older they helped me calm down, reminded me of her. It helps me keep my stress under control."

"Does being with me stress you out?" Audrey asked, half-teasing. "Because you whistle a lot when we're together."

Nathan's lips slanted in his characteristic lop-sided smirk. "Well you are a magnet for trouble," he pointed out playfully. "But that's not it. Thinking of my mom, it makes me happy." He reached over and tucked a strand of her hair behind her ear, his fingertips lingering along her cheek, fixing her with one of his intense, genuine stares that made her heart leap in her chest. "And so do you."

After that Audrey never questioned him again about his tendency to whistle. She learned to tell whether he was whistling out of stress or out of contentment, usually by the tunes he whistled. And every morning when she woke up to the sound of Nathan whistling from the kitchen, she would snuggle down into the pillows and smile, knowing she was the reason.