Ceiling fans are built using a motor that rotates the blades using ball bearings. The bearings provide very little friction and noise as the blades rotate, making the fan quiet and efficient for long periods of time. Eventually bearings can wear down, however, and the lubrication they use to stay frictionless and silent can dissipate. If this occurs, your fan will start to experience some bearing noise in the form of rubbing or humming. There are some solutions to this problem depending on the fan type.
Bearings in ceiling fans are like any ball bearing set in that they must be lubricated to function silently and at their highest potential. If the lubrication wears out, then you'll start to get ball bearings that rub against the housing they're held in. Bearings in ceiling fans are pre-lubricated when they're installed. Most new models of ceiling fan feature bearings in an oil bath, which essentially means that the bearings are fully encased in a housing that's filled with lubricating oil, so that as the bearings rotate inside the fan they automatically continue to self-lubricate. Even this method of bearing lubrication can fail after prolonged use, however. If you've had your fan for a decent amount of time and are starting to hear noise from the motor area of the unit, it may be time to lubricate the bearings. Older models of fans are designed to be re-lubricated over time and have pre-installed places to fill with oil specifically to lubricate the bearings. Newer fans aren't always designed to be lubricated by the user and as such may require some disassembly or even professional servicing to re-lubricate.
The oil to use in lubricating a ceiling fan's bearings and is very specific. Using the wrong type can clog up the bearings and ruin the fan, oftentimes requiring a full replacement. You want to find 10, 15 or 20 weight non-detergent motor oil. The non-detergent specification is extremely important, because the addition of a detergent element to the oil is what will cause the bearings to become clogged up and stuck.
Lubricating the Bearings
If the fan is an older model that is designed for periodical re-lubrication, the process is extremely straightforward. Using a small oil can, simply fill the fan's oil pan and then turn it on for a short period of time. The oil pan is filled through a small hole in the housing of the fan, usually on top of the motor. It should be fairly visibly labeled. Once you've refilled the pan with oil, run the fan at normal speed. As the bearings rotate they'll re-lubricate themselves with the new oil and soon enough the noise should disappear as the friction between the ball bearings is eliminated. If you have a new model of fan that isn't designed for re-lubrication, the process can be significantly trickier. If the fan is an expensive model or in a difficult to reach place, you should strongly consider contacting the manufacturer and having the fan professionally serviced. If you opt to complete the process yourself, you will need to remove the fan from its ceiling mount. Remove the blades from the motor housing. These are usually secured with a set of screws. Now remove the motor from the fan body and then at one end unscrew the actual motor housing. Inside you will find the ball bearing assembly and can evenly coat each bearing with the oil. Replace all the parts and once the fan is re-installed run it at normal speed until the sound disappears, signaling that the bearings have fully re-oiled themselves.