Please always backup first and perform a factory reset to roll the unit back to its factory settings to avoid any unexpected errors. By doing so, all existing presets on the BIAS Head/Rack will be replaced with default presets. Please also update the BIAS Head/Rack to latest firmware version (0.6.3.173) and see if the noise dissipates.
How to Backup?
Like all electrical appliances, electric guitars need to be grounded. A good way to check this is to mute the strings with your hand. if the noise stops, or gets quieter, then the noise may be the guitar ground. Also, if you experience the problem in one building/power socket but not in another, then the issue is most likely with the building/power socket and not the amp.
If the noise still remains after checking all possibilities, please always provide as much information as you can about the noise (how and when it occurs, the type of guitar, any pedals you use with the amp) and open a support ticket at: https://help.positivegrid.com/hc/en-us/requests/new