Listen at the door first

The engine room was a dangerous place, and Kaylee ran a tight shop. Of course, Mal knew he was the captain... but it wouldn't serve to anger the petite brunette any more than he had to. She had a proper temper on her, and he didn't want to earn the ire of her.

Or one of her notoriously surprising left hooks.

Best to suss the situation out first, Mal thought to himself, leaning the side of his face towards the door and placing his ear against the metal, near the crack he'd made in the door jam.

The rhythmic pumping of the engines, and the steam the intense heat produced blanketed a soft moaning, that, with a jolt, he realised wasn't mechanical at all, but he was sure there were gently muttered words being uttered between heartfelt exhalations. He couldn't make them out above the engine sounds though.