[The cats] took not the slightest notice of either Mr. Flay or of himself save for the sudden cessation of their purring. When they had stood in the darkness, and before Mr. Flay had removed the bunch of keys from his pocket, Steerpike had imagined he had heard a heavy, deep throbbing, a monotonous sea-like drumming of sound, and he now knew that it must have been the pullulation of the tribe.
Source: wiktionary