This is also not the first horn with a tuning trigger that I've had problems with. Without exception, almost every other horn I've ever played with a tuning trigger has BUZZED from the main tuning slide. I don't know what it is about my playing but I get a lot of resonance from the main tuning slide assembly. The two Geneva's I had buzzed and could not be fixed. The first Yamaha Neo with trigger buzzed. The second one buzzed but WAY less and I was able to mitigate it with heavier tuning slide grease to the point that it didn't buzz at all. As soon as that grease wore down though, the buzz returned. This E3 is in between the two Yamahas - I can manage the buzz with heavy grease for a time but it always returns within a day or two.
The only euphonium I have had with the trigger is my current Besson Prestige 2052 from 2007: it is absolutely perfect, it works great and it has never given me any problems with buzzing or anything. When I bought it water was leaking from the slide: I solved it by expanding the end of the tube from the inside just enough to prevent the water from escaping without preventing the slide from sliding. As a lubricant I use the red Selmer grease for slides to which I add a few drops of pistons oil to obtain the right degree of sliding: perfect sealing and sliding speed are guaranteed for a few months of daily use. On this euphonium I would not be comfortable without the trigger, as the sixths partial are terribly high.
2007 Besson Prestige 2052, 3D+ K&G mouthpiece; JP373 baritone, 4B modified K&G mouthpiece; Bach 42GO trombone, T4C K&G mouthpiece; 1973 Besson New Standard 3 compensated valves, 3D+ K&G modified mouthpiece; Wessex French C tuba, 3D+ K&G modified mouthpiece.