Does it buzz when it is not in gear? Stopping at a light in gear loads the engine a bit and can cause small vibrations.

I agree with the other poster, it is probably a heat shield somewhere.

Do you service your own car?

You could have it checked at your next service or try see where the sound comes from by looking under the car.

One way to locate a noise is to take a 4 or 5 foot piece of hose (like heater hose or garden hose) and hold it to your ear, move the other end around to find the source of the noise.

I change the hoses to my washing machine every 3 or 4 years and keep the old ones for circumstances like this.