Jack the car up, pull the wheel, spin the rotor. You most likely have wheel bearing going out as mentioned. Mine did the exact thing and this is how I found out what it was.

It will not tell you which bearing (inner/outer) it is, but while you are there it's best to do both. Should only cost $170ish. If it's way more than that look elsewhere for your work to be done. The bearing are in the $25 x 2 plus $18 for the seal give or take.