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The same type of thing happened to me when I got my new bass trombone. It was as simple as a little bit of rotor oil. Could it just be that the rotors got dried out in transit?

Check the lyre screw. Also... check all of the braces.... especially the ones that are three pieces. Make sure your mouthpiece perfectly matches the receiver taper. Could be a loose bell-rim wire, too.

Just a side note.... check the horn in various venues. If could be something else in the room.

On my euph, I've found that occasional stray vibrations are caused, in decreasing order of frequency, by:

a) something in the room;

b) a loose trigger stop rod nut;

c) a loose belly pan screw;

d) a loose grime gutter;

e) a loose lyre screw;

f) a double buzz on the problematic notes;

and that, regardless of the point of origin, the vibration always seems to originate from around the 4th valve. YMMV.

Have someone touch/hold various places as you play the pitch that buzzes.

Don't know why you're keeping the model of the horn a secret. It's like playing 'twenty questions' without all of the information. Maybe if you tell us the brand / model of the horn, someone with a similar experience could help you.

Just a suggestion.

I had a horn once with a very similar, single note buzz. It turned out to be a loose chunk of solder inside the hand grip tube. That was a tough one to track down.

--edited--

I can tell it's a York because I had the same problems. I didn't bother reading the above posts because it's TubeNet......

It sounds like it'll be a trigger issue, maybe a bit tighter would help....but feel around the instrument here and there. You'll figure it out.

I had a similar sounding vibration in a Besson EEb. Tried everything to locate it. Turned out to be an incomplete soldering of a stay plate (the discs that hold the stays to the body of the horn). It was not visible to the naked eye, but it allowed a minor vibration of the plate against the bugle part of the tuba at certain frequencies. A technician located the problem, re-soldered the offending part, and bingo!

I know you mentioned earlier that you had removed the tuning slide 'kicker' mechanism but did you also take off the finger-paddle and related spring and stop? I recall having a high-end euphonium in the shop a while back where the finger-paddle vibrated against the stop. I cured the problem by gluing a small piece of fingerbutton felt in between the paddle and the stop.