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

Not long after we left I began to notice a knocking sound in my car 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 :-)

A, this way you know that there’s a less of a chance of it having biased information or edited by other people-making false information