In my POD HD500x, I used roll of the volume of my guitar, turn the the particle verb on, hit a chord, turn the volume up and then turn the particle verb off.
This gave me a really nice pad/synth like effect that would sustain forever until I change to another patch.
I tried this very same trick/affect in the Helix and it does sustains as expected but then a low end rumble/feedback like sound comes out of the Helix and it gets louder and louder to the point that makes the effect unusable. That never happened on the POD HD500x.
I put an EQ at the end of the chain and cut the low frequencies and no luck.
I even changed the global EQ and cut the lows frequencies and no luck.
It seems like this noise bypasses the EQ of the pedal.
Does it happens on your Helix as well?