If a pedal with a charge pump is placed in front of the Superdelay, Tape Delay, or Nebulus, and the charge pump noise leaks into the Empress pedal's input, there's a good chance a high pitched noise will be heard. This is caused by the codec in the Superdelay, Tape Delay, or Nebulus aliasing down the high frequency noise.
The solution is to a) stop using the pedal that is producing the noise; b) put the noise producing pedal after the Superdelay.