After 200 miles on my LaPierre X Control 229 equipped with basic Avid hydraulic disks, I can hear the front wheel disk ventilation slots grinding through the caliper when I brake moderately! (Shimano 160 disk). I did the normal thing of re-aligning the caliper; slacken bolts, apply brakes hard and re-tighten bolts. This made no difference. Should I be changing pads to organic?
Is there any loss of braking power associated with the noise? Does it feel that the brakes aren't working fast enough? Brakes do make a similar sound when water gets on the rotor for no loss of power.
From what you are describing and because you have realigned the caliper I would suggest your pads may have become contaminated or glazed and therefore not engaging properly. Check the pads for chips, hard spots or a glassy appearance (there may be a toe formed at the leading edge of the brake which is ok). Sand the top layer of the brake pad off until it is rough and dull again.
Also check the rotor for any greasiness (make sure your hands are clean and dry). Wipe the rotor with isopropyl alcohol or other non-residue type solvent to remove any decontamination.
Hopefully this will remove any scraping sound and take you back to the turkey gobble and squealing normally associated with Avid Elixirs.
Had the same thing while riding off road over the past weekend with some overzealous braking - just happened at the trails, were working well on the way in.
I fixed them with what you did - slacken, mash down hard and re-tighten the bolts. did the same for the front discs too but that needed a couple of taps to center it proper. the rotor rubs with the slack-retighten step.
Since you mentioned that you did that and it's still there, perhaps it might help to pull the pads out and inspect them. Might be worn (though I would think that is would be only a remote chance with only 200miles done), might have dings on the pads themselves or the pads weren't clicked in proper.
Once the free stuff is done and the problems still there, I guess throwing some cash down for a pair of pads and a quiet braking experience would wipe that curious/angry look you see in the mirror :)
Add: sorry just remembered - do check the rotors too for anything wrong with it - burrs at the vents or not smooth spots.