The sound was coming from under the grate. Dale stopped. It was a low sound, not loud, with a pause in it that wasn't regular. They were on the sidewalk outside the hardware store. The grate was set into the concrete next to the curb, older than the sidewalk around it, with a smell of cool air coming up. Dale crouched down. The sound came again. A low pressure knock, then nothing, then two shorter ones close together. "Pipe expansion," Dale said. "The pipes under there are warming up. Metal expands when it warms. It has to go somewhere so it knocks." Sally looked at the grate. "It's a morning sound," Dale said. "You mostly hear it in the morning when the temperature changes." It was mid-afternoon. The sound came again. One long low knock and then silence. Dale listened to the silence. "It's finding its new size," he said. "Once it finds it the knocking stops."

They waited. The sound came again, two short and one long, in a different order than before. Dale looked at the grate from a slightly different angle. "The pattern changes while it's still finding," he said. "That's normal." "How long does it take," Sally said. "Depends on the pipe," Dale said. "Diameter. Wall thickness. What it's carrying." "What's it carrying," Sally said. Dale looked at the grate. "This one's probably water," he said. "Or air. Could be a return line." "What's a return line," Sally said. "It returns," Dale said. "Whatever went out comes back through the return." The sound came again. Lower and longer this time, with a small secondary knock after it that hadn't been there before. Dale listened to the full length of it. "It's close," he said.

They waited. The sound came again, the same as the last time. Then again. Dale stood up slowly. "It takes longer with return lines," he said. "The distance is greater." He looked down at the grate for another moment. Then he pressed his foot lightly on the edge of it, the way you'd test something you weren't sure about. The grate didn't move. It was set solid into the concrete. He looked at it. "Good grate," he said. He walked toward the hardware store.

Sally stayed crouched. The sound came again, the same pattern, low and then lower. She put her hand near the grate without touching it and felt the cool air moving against her palm. It was still going. She stood up and followed him. Behind them the grate knocked once more, low and even, like it had been doing before they arrived and would keep doing after.