i couldnt hear mine do it until i fixed my power steering pump, between 3-4k there's a harmonic, like a straw vibrating when you squeeze the end almost shut
havent figured out exactly what it is, have some remote mic's i can use to chase it just haven't gotten around to it, almost 100% sure i dont have any vac leaks, im ocd about that sort of stuff but at the same time, doesn't mean i dont either
if messing with the heater/ac director knob changes whether you hear the hiss or not itll help determine which of the vac lines behind the knob may be loose, mine will stick halfway to the floor position until i rev it up slightly to increase the vac and then itll close up