Ceiling fan makes a buzzing sound at high speeds

[El_Viajero]

It's an old-ish Hunter fan with a 3-way switch. On high and medium speeds, there's suddenly a very distinctive buzzing sound. It's a constant, electrical-sounding buzz, not a "whum... whum... whum..." sound like some fans make. The tone of the buzz is different at high and medium. At the slowest speed, everything is fine – no buzz. Could this be the switch itself or something else up in the fan's innards?

[lthenderson]

Usually when I hear ceiling fans buzzing I suspect it is the motor working harder than normal so it is off its peak torque curve. I take the fan apart and blow out all the dust from the innards and lubricate any rotating parts if it has any and the sound usually goes away.

[BudgetSlasher]

My parents have some older hunter fans, which require regular oiling.

Do you know if your fan happens to need oil additions? If the bearings are not well oil you could be causing the motor to work harder resulting in the sound you are hearing.