With my setup, I can only notice it between 8 to 13 (maybe 15) psi and in all honesty, I don't really notice it anymore coz I'm used to it already but if I listen for it, it's there - and from 15 'ish to 19psi I can't hear the spool at all over my Nizpoo zorst and the zorst isn't 'that' loud at ful throttle