The strings usually buzz a lot more when it is rusty. Since you claim that it is setup alright, the next few course of action will be to check the root of the buzz. There can only be a few problems.
1. Action too low (which also means your guitar isn't setup properly or frets are not levelled properly thus, preventing you from getting low action. However, there are also a lot of cases where the action is just simply too low anyway)
2. Frets are not level. Look at point one.
3. Strings are rusty and action too low. Yes, as your strings are uneven all over, thanks to the grime and rust, some of the frets might actually buzz.
4. Neck not straight or back bowed. (This also means that the setup is bad. Hence setup)
5. Frets unseated properly. Almost the same as uneven frets but easier to remedy.
Don't think there are anything else. I will add on to the list if I missed out something.