Between the knocking there was a sound like the roaring of lions.

Husband and wife stared at one another with white faces.

Firmin picked up his gun with trembling hands, and the movement seemed to set his teeth chattering.

They chattered like castanets.
The knocking still went on, and so did the roaring.
It had gone on at least for five minutes, when a slow gleam of comprehension lightened Madame Firmin's face.
"I believe it's the master's voice," she said.
"The master's voice!" said Firmin, in a hoarse, terrified whisper.
"Yes," said Madame Firmin.

And she unlocked the thick door and opened it a few inches.
The barrier removed, the well-known bellow of the millionaire came distinctly to their ears.