At the canal lock

An original fiction passage set in the nineteenth century.

Martin reached the canal lock before sunrise and found the water pressing hard against the wooden gates. He braced one boot against the beam and released the catch. The chain leapt over its wheel, clattering like coins scattered across a stone floor. Slowly, the gate yielded and the waiting barge began to move forwards. From the towpath, the boatman raised a hand in thanks.

Identify the phrase which uses a comparison with money to describe the noise of the chain.