Deflector hisses when Density knob isn't maxed

[falco_femoralis]

Hi all, just put together a Deflector. I get a hiss as the Density knob is turned counter clock wise. The hiss is also affected by the mix knob, with it getting louder as the mix is turned clockwise. I've checked my component values and it all seems okay. I've searched here and google and this seems to be a common issue, but I can't find a definitive answer.

Any smoking gun type thing I should look into?

Thanks,
Brian