My right knee has an interesting series of pops it used to go through when I'd ride, pop pop pop POP and then it would be done for a while - like popping a knuckle. It was annoying because I'd have to get it to POP or it was distracting. It sounds very similar and I would agree it has to do with seat height, at the right height my knee does NOT pop. It's a very subtle difference, as well, maybe two millimeters!