I’ve read quite a few posts and reviews since launch of people commenting and complaining about the PS5 slim fan noise. This appears to be common and normal. Seems like it needs to run faster and work harder to keep things cool during loading and intense usage situations and gets loud, unlike the original PS5 design where you can only hear the fan if you put your ear right up to the console, you got one of those early ones where the sticker came loose and is touching fan blades or you really need to clean your console.
I say return it if you can and get the original full size version if it really bothers you. Or get used to a loud fan.