It depends on the model. I’ve heard some ACs that are naturally pretty loud. It would help to know if it was fairly quiet before and got loud, or if its just always been loud.
A couple things to check are the rubber gasket that sits between the air conditioning unit and the roof for deterioration and check for any loose screws/bolts. If those are good, and the sound just seems to come from the fan motor, you could try an AC silencer. You can learn about those here: https://camperfaqs.com/how-to-quiet-rv-ac
Otherwise if your unit is fairly old, it may be time for an upgrade or to have a technician look at it.