I have the same problem with my Apex-i twin chamber dump valve.
Because they are piston type valves they need over 1 bar boost for them to hiss,they are still doing their job but they are quiet in operation compared to the diaphram type.
Ive stripped mine down and lightly greased the internals with a graphite grease to try to improve the noise level.
It is noticable in the car but for more noise increase the boost pressure or change it for a diap type.i can live with mine!!

 

does anyone know why my dump valve does not dump the air at low boost, the valve seems to be working but only when the boost has peaked and it is only a quick pshht, any other time all you get is wastegate flutter, i had a diaphram dump valve on a r5 gtt and it used to dump even at low boost and sounded awesome, i have taken it apart and cleaned it and even tried lubricating it but still no difference