Is it rude to visit a professors office during the summer if they have not responded to my email?
It's not rude.  It quite probably is futile.

Many faculty are not in their offices very often normally.
Many faculty are in their offices less in the summer - in particular, they are unlikely to have scheduled office hours.
During a pandemic, you might not be able to go into the building anyway.

If the professor does not feel safe meeting in person during a pandemic, they'll just tell you to contact them some other way when you show up.
Do not go to a professor's office if you are sick.  Also, do not go anywhere, except for medical care.