"Knockon" is a relatively obscure word, for which there are few commonly used synonyms. However, some possible alternatives include: to rap, to tap, to pound, to hit, to strike, to knock, to thud, or to bang. These words all convey a similar sense of physical contact or impact, which is the primary meaning of "knockon." In some contexts, the word could also be replaced by more specific terms such as "ring" or "buzz" if referring to a doorbell or phone notification. Overall, while there aren't many direct synonyms for "knockon," the word can be readily replaced by a variety of other related terms depending on the intended usage.