Make sure that nothing is connected to the unit, the headphones mix knob is set fully anti-clockwise and all processing is switched out, then listen to the headphone output. Check to see if the hiss is present whether mic, line or instrument input is selected for both channels. Also see if the noise level varies with input gain. Rotate the 'input balance' knob to see if the hiss is coming from only one channel or both. Also rotate the 'headphones mix' knob to see if the hiss is coming from the input channels or from the monitoring section of the unit.
If you do not hear the noise at all when testing the above then check your cables and signal sources for faults. If you do hear noise then please contact us for further support, letting us know at what settings it occured.