Hi Marian, I seriously doubt that there is any way to oil this, but here's a quote from Hunter's website.
Why does my fan “hum” when I installed a variable speed (solid state) control?
Most quality ceiling fans are operated by a certain type of electrical control system called capacitors. Capacitor electronics in ceiling fans create distinct speed "steps" like high, medium, and low. Variable fan speed controls, which create a speed control "range" from low to high, are usually operated by solid state electronics. Capacitor and solid state electronics are not completely compatible or interchangeable but can be used together. Because some consumers may purchase a solid state control for use on a capacitor ceiling fan, a "hum" or low level buzzing is created. For quiet fan operation, capacitor type fan controls are recommended for most ceiling fan installations.