maybe a wheel bearing. Jack the car up and wiggle the wheel top to bottom. (push and pull the top and bottom of the wheel) If it wiggles substantially, bad wheel bearing. Although, it may still be bad, even if it doesn't wiggle. also, make sure the wheel is torqued down. I had a noise in the back of my car once and couldn't figure out what it was for like a week. It turned out I had loose lugnuts. I didn't even check because it was such a simple thing, I thought there was no way I could've overlooked it. Good thing I caught it when I did. My wheel was about to fall off. Also, check all wheels. When I had that mystery noise, (it was in my crx) it sounded like it was coming from the passenger front axle, but it was actually the driver's rear wheel. Stupid phantom noises.