A very practical question. The common reasons for the ceiling fans to make a noise is
1.Loose conical cups/Canopy that covers the top and the motor components. If these light weight canopies are loose, they keep rubbing on the rod or the motor mount causing a regular repeated noise.
2.Hanging hook or the mounting plate of the fans. Fans that have mounting plate have less noise. If a hanger hook/ unapproved metal rod is used, then the noise can be increased and heard every time the fan moves.
3. Loose screws and joints. At times the screws where the blades are inserted and the motor assembly can become loose and be the source of the noise.
4. Low voltage. At times when the voltage at that particular point is low, then the fan motors struggle to work causing to noise generation. This would be seen if many devices are linked to a single circuit or point.
5. Use of dimmers or remotes. Dimmer controls are devices that are used for a smooth noiseless change of the fan running speed with the turn of the knob instead of the traditional/standard speed controls that change from 1-2-3-4 with a click each time we move the knob. The dimmers are not be used in celing fans and also it generates a low but audible noise. At times when the remote is good but the receiver unit in the fan is improper or failing, then also a noise is generated.
6. The other noises are a grinding like noise that is seen when the fan is switched on, this indicates a damaged bearing. A rattling or a wet sizzle like sounds often with sparks/smell indicates a faulty motor or motor circuit connection.
Before trying to rectify or even assess such issues, please switch off the unit. Many people try DIY methods with improper tools and a step ladder ending up with shocks or falls due to loss of balance. Better to have professional help of such issues, especially for one's that involve looking at the motor, removing dimmers and remotes. If it is just a loose canopy cover or screws of the wings, this can be tightened once the power is switched off.