The knock inside the clock

An original narrative about an invented clock-tower inspection.

Nerys climbed the tower before the building opened. Since its repair, the clock had kept a dependable rhythm: a small click, the soft pull of a chain and, once each minute, the heavier movement of the wheels. She had learned to separate these sounds without thinking.

Halfway through her inspection, a rough scrape travelled down the wooden frame. Four seconds later, something knocked once above her. Nerys looked up. As the largest wheel began its next turn, the scrape came again: four seconds, then the same blunt knock.

She pressed her palm against the frame and felt a faint shiver. A bracket must be working loose. She pictured it bending a little further with every turn until a piece of the mechanism dropped through the tower. At the third knock, she used the control that stopped the clock.

The wheels slowed. The regular clicks separated and died. Nerys waited with one hand still on the control.

Scrape.

Four seconds.

Knock.

In the new silence, the sounds seemed louder, but they could no longer belong to the moving wheels. Nerys climbed to the landing above. A board left by the maintenance workers had slipped from one of its hooks. Each draught pulled its lower edge across the stone wall and then let it fall back.

When Nerys restarted the clock, its old sequence returned, orderly and exact. This time, she heard nothing wrong with it.

How does the writer lead the reader to share Nerys's mistaken interpretation of the sounds and then reconsider it?