Sometimes a hiss in the audio signal can indicate an impedance mismatch. So, you could try a different pair of headphones to see if you get the same sound, though that is just a guess. If your headphones have a volume or level control, try turning down the level on the headphones and then turning up the volume of the keyboard.
Similarly, if you get the hiss when using an amp, or when hooking the keyboard up to a computer to record to a DAW, you can try adjusting the levels. When I first started recording to a DAW, I had a constant hiss in the background, and I just attributed it to a low-quality analog-to-digital converter. I tried all sorts of settings on my DAW (Audacity) to try to eliminate the hiss, and while these efforts were somewhat successful, they did not get rid of all of the hiss, and would often alter the actual sound of the music somewhat.
Then, when doing some internet research, I stumbled on the idea to adjust the levels. I turned the input level on my computer down, and then turned the volume of my keyboard up, making sure not to go "into the red" on the level meters, and the hiss was gone.