Get up on the roof and take the cover off. There are some baffles in there that can have loose screws etc. Also a popular place for birds to make a nest.
My rear Dometic was noisy until I changed the gasket, then it quieted down. I expect the old gasket allowed the mounting bolts to loosen. Do not over tighten the mounting bolts or you'll have a leak. They take something like 40 INCH lbs of torque.
Jerry & Jeanne
1996 Safari Sahara 3530 - 'White Tiger'
CAT 3126/Allison 6 speed/Magnum Chassis
2014 Equinox AWD / Blue Ox