I, too, assumed this thread was going to be about a computer fan and I was ready to tell you to blow it out with compressed air (which is what I do every month or so, and it keeps my computer way quiet).
Personally, I find nothing more relaxing in the summer than laying in bed with the fan on...though the ceiling fan I use now has some weird sounds sometimes. I figured out that the glass piece covering the light bulb comes loose sometimes, and makes a loud vibrating sound. Don't know if you were talking about a ceiling fan, but if you were, you could try tightening those screws.