FabFilter

I am mixing a song in pro tools with saturn on my mix bus, and it sounds great while the track is playing, but it gives a really annoying high frequency hiss when there is no audio playing. When I bypass the Saturn plugin the hiss goes away, so that's how I isolated the problem. Any ideas how to get rid of this annoying painful hiss?

Put a noise gate (FabFilter Pro G?) after Saturn in the bus.

But perhaps the hiss is happening because one ore more tracks are armed to record, or with input monitoring turned on, perhaps even with an instrument or mic plugged in. Turn off input monitoring and disarm all tracks!

What are the settings in Saturn? I'm getting a noticeable hiss from that plugin only in case I would do some sort of heavy saturation in the higher frequencies and it's getting louder when boosting the treble and presence bands. After I'm generally very carefully when doing saturation in higher frequency ranges this didn't bother me yet.
But maybe in your case it's a bug!?