i'd say automate the volume or envelope. both these options give you way more control and it's a better habit to get into. i'd probably go with the envelope, and hell, why not automate the filter as well, probably set to an lpf.
much of what we percieve to be ominous, psychoacoustically speaking, is high frequencies creeping in at hearing's noise floor, especially 1k to 3k frequencies. that's the reptilian front lobes being stimulated. ever wonder why quiet rustling sounds are so unnerving to large mammals such as ourselves?
I don't know...the slide button never seemed to work for me..