You have to be hearing the compressor surge. I run the same BOV and you can not mistake the sound it is a very loud unit. I had a slight problem when I first installed it. I pinched the o-ring that goes in the mounting flange and it was making it hard for the BOV to open as soon as it should. once I got that fixed everything is great.
Start the car open the hood give the car a little gas and if the BOV doesnt open alittle when vacumm hits than something is wrong. could also be the spring as mentioned above.
Also my fox runs around -18 vac, maybe you have a little leak unless you are running a different cam.
88lx Conv. 200K mile stock longblock. Trickflow intake, 42lb inj, Pmas protube,TKO500, 3:27, pypes muffs, Pa Racing tubluar K-member, DIY single turbo setup. Summit aluminum rad, Crane Hi-fire ign, subfames, 17" sc saleen's, Saleen ground effects, SCT tune by Dynosport. 380HP 435ftbs.