I'm a new user and had the same 4CM question. After troubleshooting with a "New Tone" patch with nothing on it except for the FX Loop, I found two sources of interference.
1. The FX Line/Stomp switch - Line was too much. Switched to Stomp and that removed some of the interference (hiss).
2. USB connected to my PC - This was the main source of noise. After unplugging the USB from the HD500X, there was silence and the 4cm sounded pretty much like my amp. I tried several USB cords with the same results (they all made noise).
Thanks!
