New Helix user here. Having a similar issue. Didn't have the issue until I started using XLR output and "mic in" for my microphone. With the guitar alone using guitar in and the 1/4 output, I hadn't noticed an issue.
I checked the phantom power and it was already off.
I changed the XLR output to Mic instead of Line, and as the original poster said, it did decrease the noise by probably 80%. But there is still a noise, and I feel like it should be silent or at least closer to it.
If I unplug the cord from the XLR output, it goes away. There is then still a tiny (but really very small in comparison) noise left, which goes away if I then unplug the 1/4 output that my guitar is using.