I know this is an old topic, BUT I think I have found one possible cause of the problem (at least on my case), and I hope this helps someone.
My setup
Helix connected using 4CM to a rack full of gear (mostly preamps).
Signal Path:
Guitar > Helix > Helix FX send > Outboard Preamps > Helix FX return > Helix XLR -> Audio Interface
The Audio interface is connected to a laptop via a powered USB 3 hub. I have
The Computer gear, Audio interface and Helix are all powered by a Furman line conditioner. The outboard rack gear by another Furman. Both Furman line conditioners are currently connected to separate power points but this makes no difference whatsoever. I have tried all possible combination of power points, house circuits, all gear powered by the same Furman device, etc.. none of that matters.
I get noise (and a LOT) when the helix is connected to the powered USB hub and the outboard gear are connected. I also get noise (like digital noise) when the Powered USB hub is connected and the outboard gear is not, although is not as much noise. The first sounds like a ground loop but it makes no sense.
The solution in my case was simple. I just connected the Helix to a standard USB 2.0 port. In my case, this was easily achieved by connecting the Helix directly to one of the laptop's docking station USB 2 ports. I was also able to use the USB c port on the laptop using the Dell USB-C to USB-A 3.0 adaptor with the same success. I tried another two powered USB hubs and all cause noise, and this are not cheap hubs. I did not try with non powered USB hubs for lack of devices.
I admit this might not make much sense but I hope this helps someone.
