I had a similar thing happen to my 2000 Forester.
My conclusion was that some corrosion on the wheel/brakes caused the cyclic scraping sound. Boy was it scary.. it sounds nasty. But after driving for a couple miles, the sound went away and has not returned.
There were a few rather moist/humid/rainy days and the car sat in the driveway for 3 days or so. I think it was just enough to corrode somewhat.
On the other hand, be cautious if you hear such noise. I had a sears auto center rotate my tires once. They incorrectly mounted my front right tire.. it was not on flush.. and also made a nasty scraping noise. I knew darn well what it was as I had just left the place when the sound started. Never did I go back.
Oh.. I should also mention they denied that was the problem. After 'looking' at the car they said there was nothing wrong. Interestingly.. the sound stopped after leaving the place.. and I watched what they did carefully. They remounted the very same tire.
I no longer trust sears, nor many of the 'speedy' type oil places. I have had too many amateur mechanics there that just do things 'fast' without caring about the quality of their work. Many busted splashguard fasteners, oil and ATF fluid overfills, I had enough. The convenience wasn't worth the pain.
*now that I have strayed from the topic.. i'll stop*
-Gary