Etiquette Question - Professor asked if I could meet after X time on Y day. I emailed to confirm a specific time but he has not responded
No, it would not be rude to knock on his door. He's already said the time is OK! However, I would not have worded the email asking him if [time] is OK - I'd simply say something like "alright, I'll be there at [time]", which saves him from having to respond.

If he indeed rethought the offer, it'd be quite unethical of him not to tell you before you visit him. But you knocking on his door won't hurt anyway: the worst that can happen is that he tells you then that he's rethought the offer.