There are sources that will tell you sleeping under a fan or with the fan blowing on you can cause dry mouth, nose or throat. However, there are people who have slept most of their whole lives using a fan to help them fall asleep and drown out the noise, using the fan as white noise who never have an issue with dry mouth, nose or throat. As for mucous development, as long as you have a good diet that is high in whole foods, fruit, and veggies or leafy greens, you have a much lower risk of developing any mucous in your throat or nose while sleeping.