This is caused by the lamp filament vibrating as the dimmer rapidly switches the lamp on and off. Lamp hum is generally noisiest at the mid-range [50%] dimming level. If this happens use rough service lamps [sometimes called garage door opener or appliance bulbs], physically smaller lamps, or lower wattage lamps.
Upgrading to a quality, or even electronic, dimmer is a better solution.
im going to replace the dimmer with a better one. the one thats installed is a cheap dimmer from home depot. theres skylark dimmers in the other parts of the house that dont do it