Agreed with @erik_squires that diyaudio is likely to be of more help with this issue.
Perhaps this thread is somewhere to start: https://www.diyaudio.com/forums/diyaudio-com-articles/163575-audio-component-grounding-interconnecti...
When dealing with ground loops it can be useful to just unplug everything and try to eliminate possible sources. Do you have to same issue if the pre is taken out of the loop? Do you have equipment plugged into multiple outlets?
Good luck, ground loops can be a bitch...
Perhaps this thread is somewhere to start: https://www.diyaudio.com/forums/diyaudio-com-articles/163575-audio-component-grounding-interconnecti...
When dealing with ground loops it can be useful to just unplug everything and try to eliminate possible sources. Do you have to same issue if the pre is taken out of the loop? Do you have equipment plugged into multiple outlets?
Good luck, ground loops can be a bitch...