All dump valves open when no throttle is applied when throttle is applied it then closes and opens fast when you release causing the thsssssssshhhhhhh lol so it's normal stg3 there is no need for a dv it's purely a personal thing. I like them cos when I was little I used to be amazed by the sound of a cossie or rst with a dv lol again not everyone likes them but I do