I have a small window fan in my workshop. It's one of those old metal expandable units that fits any width of window by moving the sides. The fan blade is only about 9 or 10 inches. The motor is a shaded pole motor, similar to the motors used in the old record turntables. But this fan sings. It stays at a constant speed, but the tone varies. It can be heard 300 feet away. It's not a grinding sound and the blade is not rubbing the housing. It's just purrs along, while changing in pitch. Sometimes it's pretty constant, then suddenly the sound will go up and down rapidly for a half minute or so. At first it was sort of annoying, but after awhile I have grown to like the sound. Its sort of soothing and varies like the waves on a lake.
However, just because it's a perfect fit for the small window in that shed, I decided to take it apart and make sure it was oiled. The bearings are all pretty tight. I oiled it's bearings well and reassembled it. I was surprised that it was much quieter for the first ten minutes. Then it went back to singing.
Just curious if anyone knows why it makes these sounds?