The website doesn't mention any limitations the demo is having, and since a demo is always limited in some way so you can buy it, I'm fairly sure this noise is just a demo limitation.
In order to test this, create a new instrument, assign the vsti to it, go to the instrument tab and press a note to initialise a sound. Let it naturally fade out so you hear nothing.
I expect this sound to start sounding too. If so, its a demo limitation. If it were to be something from modplug, you could find out by telling us if other plugins (preferabelly free ones) also have this sound once in a while. If not, you should contact the plugin developers instead.