I finally got sick of it and started taping off joints in the bugle to see if I could find it without spraying each joint with soapy water or even fully immersing the damnable beast.
After 30 minutes or so of putzing around, I discovered that one of the corks in my 5th valve had flaked off and was allowing the stop arm to rest in an over-rotated position, badly enough that it was causing the horn to sound like the water key was open.
These valves are removable, so that I would be able to install them in an awkward position and location. Unfortunately, this makes it difficult to inspect the rear alignment marks or the corks themselves while the valves are on the horn, and I have not taken it apart in some time. Soooooo… now I feel a bit stupid.
At least no one had to hold my hand while I fixed it.
