I'm guessing you have a class A on a Workhorse chassis. The fans on those are notoriously noisy. Don't know why..the new Winne we just got on a Ford chassis seems to have a very similar fan (huge plastic multi-bladed thing) but it's far quieter. I'd quieted our Fleetwood Flair (on Workhorse w/8.1) considerably by pulling the doghouse and insulating the bottom with Summit Racing's brand of heat/sound deadening mat. Also replaced the doghouse to floor seal with new heavy black sponge rubber from Home Depot. Made a huge difference. Took 2 kits. Clean the underside of the doghouse best you can to help the stuff stick..
https://www.summitracing.com/parts/sum-879014/overview/