But at my back in a cold blast I hear The rattle of the bones, and chuckle spread from ear to ear

The Fire Sermon begins with a description of the river Thames crossing the city of London. By means of a negative enumeration “the river bears no...” the actual landscape ends up sheltering rats.
This line and line 196 refer to a very popular poem by Andrew Marvell, to His Coy Mistress: “But at my back I always hear / Time's winged chariot hurrying near;”

a river of people