wild stab but..
i dont think the sound is coming from the dump valve, its a recirc, so is silent. your hearing the sound of the induction/filter releasing the air it was sucking in just before you let off.
as the forge works better at passing air than the standard one, theres more flow bypassing the turbo and being released from the filter.
i had this dv on my saab 900 turbo. people use to think i had an atmospheric dv which it didnt, infact i couldnt use an atmospheric dv as it would over fuel.
(i think recircs are better as you dont get a stupid sound @ normal driving conditions)