That is the one thing that really bugs me about disc brakes.
There are a few ways correct it, some free, some not. One is to spin the wheel and figure our where it rubs. Then bend the rotor slightly by hand, with a tool of your choice, or with the proper tool like this:http://www.parktool.com/products/detail ... &item=DT-2
The professional way is with a wheel truing stand and a rotor truing attachment such as:http://www.parktool.com/products/detail ... &item=DT-3http://www.parktool.com/products/detail ... em=DT%2D3i
If you have brake calipers that can be moved laterally with one or more bolts, you can loosen the bolt, spin the wheel so the rotor centers the brake pads, then hold it in place and tighten the bolt. That worked perfectly on one of my old bikes that I couldn't get rid of the noise by bending the rotor. If your calipers are shimmed, you have the nightmare of trial-and-error to adjust the spacing to center them.
It's really annoying when you think you've fixed the noise, and then it comes back after you've been riding for a while.