As much as we appreciate dimmer switches, they can make low-quality light bulbs flicker and buzz when you turn down the juice. Often the problem is with the bulb and you just need to swap it out for one with a heavier filament. I'd suggest switching to a newer, energy efficient compact fluorescent bulb. Or at least install a long-life replacement.
On the other hand, if you still have trouble, it may be with the dimmer switch. If you're confident using most household tools and know how to shut off your power, you can replace the switch yourself. If this sounds difficult to you, just call in an electrical contractor because it's a relatively quick and inexpensive job.
There are two wiring configurations for dimmers-- single pole and three-way. According to Home Depot, you should opt for two switches, so you can dim the lights with one switch and use the other for on and off functions. Set it and leave it be.
Replacing the Dimmer Switch
Stop. Don't pick up a tool until you've turned off the circuit at the breaker box. Whew. Now grab a screwdriver and remove the set screws from the switch plate at the wall. Pull the switch away from the wall box, being careful to not pull out any wiring. Smart homeowners test the wiring with a voltage tester to make sure they're disconnected from the power source.
There are three wires--black, white, and green (or copper). The ground wire is attached to the green screw, so remove it last. With a three-way switch, you need to locate the common wire. It's usually a different color from the rest and may even be labeled "common." Mark it, then disconnect all the wires from the old switch.
Now you work in the reverse order, attaching the ground wire to the new dimmer switch and cap loose wiring ends with a wire connector nut. For a three-way dimmer, connect the black dimmer wire to the common wire, attaching the other two black wires last.
Now you're on the home stretch. Carefully feed the wires into the box and position the dimmer plate flat against the wall. Screw in the plate and step back. It's done! Head on over to the breaker box and turn on the juice. Presto! The bulb lights up, adjusts with the dimmer, and there's no buzz.