I mainly hear it from the front. I have installed the slotted and drilled rotor on both the front and the back and recently swapped sides based on someone saying that the rotors are directional.
The slots are curved and now face "back", meaning, as the rotor turns clockwise, the slots are being "blown back" like this )
Before I had the rotors such that the slots were "facing forward" like this (
Hope that makes sense.
The sound didn't change.