that spring is too heavy, you need to source the one that matches the amount of vacuum you pull at idle. so IIRC the white or plain spring. ask hustler and/or shueind.

I have a feeling youre just hearing flutter and not surge....but it's possible the bov isn't even opening as that yellow spring is too heavy.

It flutters for a split second.. thats what you're hearing... same noise as the full flutter however only for a split second until the bov opens.

As said your spring is too tight. It should stay open at anything less than 15"...idle is about 20-25".

I put a clamp on the line and heard the same noise, but far more drawn out since the valve wasn't opening at all. According to the spring selection chart, I draw slightly more vacuum than my spring is rated for. I'll remove the shim and see how it goes. I suppose a hissing noise at idle isn't the end of the world.

hissing noise at idle tends to be the TB bypass screw out too far.

I took the last washer out and it's a little bit better. I wonder if I screwed it up when shimming it. There's a piston that goes into a cylinder and I suppose it's possible that I may have damaged those when forcing the two halves back together. Everything looks fine but I'm not sure what else could be causing this problem. The thing blew off with great vigor and enthusiasm when I first installed it.

Like i said before, it sounds like you just couldnt hear the surge with the filter on the compressor. The intake tube tends to amplify the surge sound.