Dave Smith Instruments SYNTHESIZERS

I am still learning the full depth of this synth and all it's signal path possibilities - so this 
might be something obvious to someone else.

If I have the VCA set to 100% envelope, and set a short release time, I find that I can still 
hear the oscillator humming in the background as a constant after the release time has 
passed.

I checked to see if there were any delay or feedback settings that might be causing a small 
amount of sound to be looped back into the signal path, but this stuff is all set to zero.  
the filter is not self oscillating - but there is sound, quiet mind you, but still sound.

What could be going on here?