2009 Ford Focus rear end grinding noise?

Hi I have a loud rear end grinding noise that starts
at about 15 mph. Sounds like rrrrr. I replaced both rear hub assemblies. New Shocks on rear. New rear sway bar link bushings. Took of rear bumper cover. Sealed foam liner to bumper with silicone and put bumper cover back on with new hardware. Still no change in the noise. Please advise