Question about chromatic harp

The Hohner 64 os the granddaddy of a ll 16-hole chromatics. It's what Little Walter played. So it's not a bad model.

I hear the airiness in the top octave, not so much in the third octave.

Air leakage can be caused by a number of things:

• Loose MouthpieceThe Mouthpiece?Slide assembly is fasten with two screws at the right and left. Too tight and the slide won't move. Too moose and you get air leaks. Test the tightness of those screws. Try loosening each one, then tightening it until the slide binds (won't move). Then back off in quarter turns until it moves freely without sticking.
• Loose ReedplatesThis can be tricky, depending on the period your 64 was made. Older ones use long pins (like finishing nails) to fasten the reedplates, that go through both reedplates via holes in the plastic comb. Newer ones use screws.Hoping that this one uses screws, again, loosen, then re-tighten the screws to just *finger* *tight* - too tight will make the reedplates buckle and cause air leaks betwen the reedplate and the comb. Tighten starting in the middle and work outward toward the left and right edges of the plates.Pins can be tightened with a nail punch and a very small hammer. Better still is redplacing the nails with screws, but that's its own discussion.
• Missing valvesThe top octave on most chromatics is typically unvalved and doesn't need them for airtightness. But valves missing lower down can cause significant leakage. With the covers off, you can check the outside valves on the blow reeds, which alternate left to right with the draw reeds. A chromatic will actually play OK without the outside valves and will bend more like a diatonic, but missing inner valves (on the draw reeds) can cause significant air loss. To check those, you'd need to remove the reedplates.

My money's on either a loose mouthpiece or loose reedpaltes.

Those faux-alligator plastic pouches were in use in the 1960s=70s, and the mouthpiece and button shape and pinned-on reedplates are consistent with that period.