Thanks for your help. I have a Keeley pedal compressor and I put it in series with the Helix and Marshall. I turned the compression setting to minimum and turned the gain control down, it got rid of the noise. I think I need to attenuate the input level of the Marshall, just turning down the patch volume makes the patch quieter but I think there's just too much gain on the Marshall return and its boosting up the noise floor of the Helix outputs. I figure it's a bit like turning up the amp to full and rolling back your guitar volume, not recommended. I'm Going to rig up a simple pot in a box, put it between the two and see what happens. I'll post the results!