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Indeed, the stethoscope is a good idea.

Another is to get a fairly new pencil, nice and long, with a full eraser on it. Go under the piano while someone else plays sustained chords, and firmly touch the eraser to various points along the ribs. Likewise up top on the bridges, and anywhere else that looks suspicious. If the noise stops, you've found the source.

Whatever happened to Jesp1? (Did i just hear a hissing sound??)

Funny, my snake makes a piano sound!!!!

Excellent!

I want to thank everyone for their suggestions and insights! Loved the big snake! I realize locating the problem could require an ongoing exhaustive investigation. I've not entirely ruled out sympathetic vibrations outside the piano, but I've moved and repositioned a lot of objects within the room with no consequences. I've also ruled out any of the ribs or struts as suggested. I really liked the idea of using a mechanics stethescope and think I will purchase one soon, because right now I only gererally feel I know where the sound is originating. Neither the dampers nor loud pedal have any bearing on the sound that I'm able to determine. The loudness of the hiss is not consistent in all the keys within the range wherein the problem exists (about 2 1/2 octaves in the middle of the keyboard). Perhaps 4 or 5 keys are more pronounced that others, expecially c below middle c. No one cited the bridge as a possible explanation, as is flattens out with seasonal humidity drop? Just wondering. As much as I would be thrilled if change of season eliminates the problem it will also be a bit of disappointment if the cause remains unidentified.

Sometimes, all it takes is a tiny object such as a pin or a few grains of sand etc on the soundboard, beneath the plate somewhere, to vibrate sympathetically and produce a sound that can be described as a hiss.

In one case I remember with a client's piano, a hissing buzz turned out to be a bit of dirt laying in the sheet metal heating duct below the vent which was not so far away....

When the sound first appeared I had a suspecion and was convinced about it being debris or something small on the soundboard, since I did have some work done on the treble bridge just a week or so earlier, which involved bridge notching and and sanding, but alas, I was never able find anything. I guess that would have been too easy.

If it's something like that few grains of sand, perhaps blowing it out with compressed air would help. I have a big air compressor that really blasts the dust out from under the plate.

I'm not sure you can rule out sympathetic vibration. Lots of things, both in and out of the piano can vibrate sympathetically. Could be a loose screw/washer on the sostenuto knife assembly. Could be strings touching each other or the plate casting somewhere in the backscale area. Loose hinges or hinge pins, fallboard locks, damper felts (Trichord variety) pulling out of the strings upon play. Does it make this sound only when a note is played? If it happens when the sustain pedal is depressed it is likely the trichord damper felts. They would be confined to about two octaves in the middle of the piano. I know you said you have eliminated the dampers as a source, but have you really? ;-}

Yes, I hear the sound both when the sustain padal is depressed or left alone. As a test to eliminate the damper felts, I have mutted all the strings with wet towels in plastic bags and physically lifted the one damper felt on the one trichord being tested in the cited area, so as to have it completely clear the strings and still get the hiss/buzz.

Dale, I think I'm about to reconsider my position on the dampers!!! I was striking G3, which has been one of the more offensive notes, while standing on the left side of the keyboard and I tried to listen where the hiss/buzz was coming from and I realized it was closer to the bottom end of the keyboard rather than near the note. I pushed down the bass dampers fairly hard with my left hand, holding that position for a couple of seconds, and proceeded to move up the range of dampers, doing the same thing until I got to G3. When I was done...NO MORE HISS/BUZZ!! Apparently when I had attempted to mute the dampers previously I wasn't applying enough downward force to the damper heads, which down begs the question...was it the felts or something else (downward pressure on the stings or loose damper heads)?