Droning Noise at 47 mph and up on 2007 Lincoln MKZ

Smooth quiet ride until I reach 47 mph, then a droning noise, sounds like a bunch of bees, which starts and pitches higher and higher at faster speeds. Car recently purchased used, with only 20,000 miles on odometer. Seems noise is coming from under engine.

Asked by for the 2007 Lincoln MKZ