Gutten Morgen!!
A dump valve will ONLY blow if it has pressure there ie boost, as other wise it has nothing to let out so to speak.
When you go from driving(on boost) to let off the accelerator the vacuum overcomes the spring and so allows valve to open to atmosphere(or recirc)but if no boost there you cannot get any sound.
I have a Diaphragm type in mine, group a that is very loud, I even drilled the holes in the body out bigger to make louder.
tabetha