lightweight

I just bought an Ironhorse bike and everything is smooth but the rear mechanical disc brake(this is my first bike with disc brakes). They are Hayes IH2 brakes. The disc looks centered and the brake is mounted tight but if I hit a bump or pick up some good speed there's a strong "twanging" sound, like the disc is vibrating and hitting against something. When I was checking the disc to see if it was loose I flicked it a little to see if it would vibrate any. Away from the brake it just made a nice hum but close to the brake it sounded like it was hitting against something. The disc gaps look good and the disc seems almost perfectly true so I'm pretty confused as to what's causing the noise.

Any way to get rid of this noise?

Thanks