Blow off valve without a turbo?

i have a buddy that has a 01 honda civic. a guy drives around town in a 98 honda civic and everybody thought he had a turbo because when he reved it up or shifted gears there would be a loud blow off noise like turbos produce. he recently admitted to not having a turbo but having a blow off valve that you can buy that hooks up to your throttle and releases at a certain RPM. anybody know what this might be called or where to find this particular type of valve? I told my buddy i would try and help him out and see what that guy was talkin about. thanks for any help