Question

Not long after we left I began to notice a knocking sound in my car. _______, there had only been the purr of the motor.

Answer 1

Not long after we left I began to notice a knocking sound in my car. Before, there had only been the purr of the motor.

Answer 2

Not long after we left I began to notice a knocking sound in my car
`Before` there had only been the purr of the motor....... So the word you're missing is before which makes sense in the sentence... hope it helps :-)