Mostly.
So I was practicing, as I said, and at the end of every high Db or low C, there would be a light rattle that lasted about a half of a second. It was loud enough to hear but too short to locate. I moved into two different rooms as I thought it was sympathetic vibrations from things on or near the walls. It was probably not audible in the concert hall, even by our bass trombonist.
But it bugged me to no end. I suspected that the worst joint on the tuba to re-taper and to correct the fit in the ferrule must have a gap that I fed too much solder into, causing there to be a solder dingleberry, and that was flapping around inside the beast.
For the first 20 to 30 minutes I thought I had knocked my bell wire loose enough to vibrate sympathetically. My wife and I used an engine stethoscope, I moved around, aiming the bell at different surfaces, I pecked at the low C short and crisp, over and over, to have as little tuba and as much rattle as I could generate. No good. It was audible but very faint. But it *was* there.
I peeked through the bottom valve cap vent holes to see if one of the springs was out of its recessed home. It all looked fine to me.
After an hour of this work bore no fruit I gave up and decided to look for it after this upcoming set of rehearsals and concerts.
Long story short: It was the 4th valve spring. The piston end of the spring was slightly unseated so that the coil was banana-shaped and would vibrate on certain notes. The plastic coating was preventing me from localizing this vibration. It was very faint, and the metallic aspect of the sound was being attenuated enough by the plastic coating that it sounded like it is was coming from INSIDE the bugle somewhere. All I had to do was remove and reseat the 4th piston, et voila! no more noise.
Boy, I felt stupid!
