Well, having had both my water pump and alternator replaced (circa 100,000 miles), plus knowing there are mystery resonant noises with all the bloody heat shields on cars these days, my best guess would be the water pump. Have you noticed any pink seepage near the pulleys on the passengers side? I didn't have any visible signs myself. I simply demanded that my mechanic replace mine (against his wishes). Once off, when you spun the magic wheel, you could hear that the spindle was grinding. The customer is always right.