I have found no surefire way of keeping rodents out. Sometimes mothballs or pepermint oil work, but you probably don't want either near your fan. Using the vehicle frequently will discourage them. If you're parking in a garage, setting traps near the car will help too.
I planned to take my car to the dealer under warranty when I heard the noisy fan, but they were nice enough to tell me that if it turned out to ba an animal, it wouldn't bec overed by the warranty. I can't blame them for not covering it, but they really should have warned you in advance.