The blow off valve is not part of the wastegate. Some cars like the old GT-4's never came with a blow off valve. The blow off valve realeses air preaserue when you lift off the throttle so the resulting shock wave of the air hitting the throttle plate dosent bounch back to the turbo and atop the blades from spinning. The wastegate allows air to bypass the hot side turbine to limit the amount of boost the turbo produces. The only way to cange the way a wasegate sounds is to get a divorced gate and dump it to atmosphere.