Actually, this is a major symptom of using either the wrong VCA or wrong envelope behavior. When you use linear VCAs and/or linear envelopes, this can happen because our ears aren't linear with respect to "apparent loudness". We hear exponentially as far as volume is concerned, and the Decibel scale reflects that.
The actual fix here isn't a noise gate (sorta-kinda)... --Lugia
Lugia, thanks for the info but in my current patch there is no VCA after the filter, and the noise is present when there is no envelope going at all. And yes a noise gate gets rid of it. As I said, a VCA after the filter could also take care of it, thank you for elaborating on that. I'm aware of the rule of thumb you site in regards to linear vs exponential VCAs, I would just point out that as with any rule, there are exceptions. I use my ears and do whatever works for the current situation. A lot of my VCAs have adjustable response curves, so it's easy to go back and forth between linear and exponential without having to use another VCA.
The Frequensteiner sounds great but is a notoriously noisy filter, that's the only reason I mentioned it.