Dave, I feel your pain. No ... wait ... that's MY pain. Never mind.
A couple of months after I got my Wessex Eb Champion tuba, it developed an annoying buzz -- but of course only on one pitch. As we all know, these are maddening to zero in on. My approach is to apply "the laying on of hands" over the entire instrument until I find a place that makes the buzz stop. On a compensating tuba, that can cover a lot of territory, and be quite tricky since if you hold it differently, you may not get the buzz. It turned out that the solder joint on a tube brace had given up the ghost -- but of course, not in a way that was even remotely visible. So much for Chinese factory electro-soldering (though I'm told the problem isn't only with the Chinese). A few moments with the small torch fixed it.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)