If you knock around or knock about with someone, you spend your spare time with them, either because you are one of their friends or because you are their boyfriend or girlfriend. I used to knock about with all the lads from round where Mum lives They were knocking around together for about a year. = hang around
If someone or something is knocking around or knocking about, they are present in a particular place. There were a couple of decent kits knocking around, but this wasn't one of them!
If someone knocks around or knocks about somewhere, they spend time there, experiencing different situations or just passing time. reporters who knock around in troubled parts of the world I know nothing about him except that he knocked about South Africa for a while