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Assuming the vibration isn't coming from another object within the room... note: I'm not a repair guy!

Some common causes of the sound you speak of could be with the bracing. I would check all the braces to make sure they are all soldered down real good. If any are loose, it could be the brace and the instrument vibrating against each other. Sometimes two tubes are (supposed to be) soldered together - ie there's no actual brace - and if that solder has loosened, that could be a problem as well.

I might also check your valve springs (if it's a piston tuba).

A good idea - get somebody else to play your horn... and play those F's to see if it is you or the horn. I don't know how it could be you, but it's always a good idea to clear that possiblity (it's reall easy to do, right?) before going through all sorts of technical/repair steps.

Am I missing a very obvious problem?

Does it have a lyre holder with a screw that needs tightening? If so, that's a very obvious problem.
If not then:
Never mind.

Everytime I play an A chord on my guitar, ten cars outside slam on their brakes at the same time.

zoro wrote:

tubatooter1940 wrote:Everytime I play an A chord on my guitar, ten cars outside slam on their brakes at the same time.

If 10 cars outside slam on their brakes, does your guitar play an A chord?

Might just be ten Accords

zoro wrote:

tubatooter1940 wrote:Everytime I play an A chord on my guitar, ten cars outside slam on their brakes at the same time.

If 10 cars outside slam on their brakes, does your guitar play an A chord?

Three pounds of flax ...

TubaTinker wrote:

zoro wrote:

tubatooter1940 wrote:Everytime I play an A chord on my guitar, ten cars outside slam on their brakes at the same time.

If 10 cars outside slam on their brakes, does your guitar play an A chord?

Might just be ten Accords

Would that equal one Accord-ion?