Forgive me, because I don't know the technical terms for it, but my friend had the same problem where he heard air escaping on the neck neat the octave pip; and as it turned out, the brass around the octave pip had to be soldered as small holes had literally been eaten away by corrosion. He could only tell by making the ends airtight, sealing the octave hole and putting it underwater. Sure enough- little bubbles.
A2 hiss is a common problem, caused basically by the compromise position of the neck pip. Ideally the register hole should be at the displacement antinode of the octave note. For the neck pip this is C2. Down at A2 you've moved off that antinode (where the air moves the most but there is no pressure change) to a point where there are pressure changes happening. This does not kill the note, but it does mean you have air moving in and out of that little hole, forming turbulence that hisses--at least that is what I guess is happening to form the hiss.
Stretching nylon mesh probably prevents the turbulence, by slowing down the air or breaking up the coherence of the flow at the edge.
I wonder if rounding the top edge of the pip hole would eliminate the problem. Has anyone ever tried this?
I've never had this problem with an alto, but I owned a brand new Mark VI tenor that had this problem from day one. It always bugged the hell out of me. I kept that horn for almost 30 years before I sold it, still in practically new condition. I think the reason I hardly ever played that horn was because of that annoying hiss, and no one back then seemed to know how to fix it.
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