on Your PC have you unchecked on "Listen to This Device" ?,sometime Line in,aux in,stereo mix or whatever analog input is enable and the mix are up,it do create a hissing noises,and have you disable the Normalizer or Loudness Equalization?
My monitor is a Benq Zowie RL2455 Model, and I connect my headphones directly into the headphone socket, I have another headphone and I dont get a hissing sound on it when I connect them to the monitor.
Depending on how sensitive the headphones are you can hear noise in some and not in others. Really no fix for it past just not using the monitor port and using the PC one without the issue. IEMs are a lot more prone to this since they are built to be easy to drive and for low power devices so are a lot more susceptible to extra noise.