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Probably something wrong with the horn. Give it a good bath and snaking. Check all the water keys to make sure they look OK, and try playing that note while holding or clamping down all of the water keys. How is the note when fingered 1+2 or 3?

The fact that it's just that one note leads me to believe that there is something wrong in one specific spot that only affects a node for that note. This could possibly be an leaky waterkey, or a piece of debris in the horn. Try cleaning and snaking the horn first, especially the leadpipe.

When you say that when you lip it around to get rid of the fuzz, but can't get it to a loud volume cuz it will crack again, what do you mean by crack? It sounds like you might have a double buzz issue, which I would ask someone else for advice on how to fix it.

What happens when you play the D quiet, and then crescendo as much as you can?

Have you tried buzzing the D on the mouthpiece and then placing the mouthpiece back in the receiver while buzzing ( with appropriate fingers down)?[/i]

My 184 recently developed a similar problem... after almost 30 years of outstanding service. Verified by other players, too. (which is something you may want to do; see if anyone else has the same problem with it) So I gave it another bath and snaking. Nothing in the tub as a result, but now its back to its old self! Go figure!

You've said twice that you took the MP out of the Kanstul and put in another BBb horn and the D was fine on the other horn. All that tells me is the MP works for the other horns. Have you tried a different MP (different shape/style, not just another MP of the same type) in the Kanstul? Have you had someone else play the horn and have the same results?
I don't think the MP is the problem but I would at least try a different MP first. It sure does sound like something is stuck in the horn or the valves are not alligned properly or it's leaking.

Good Luck!

I've seen some 'brand new' horns with some pretty strange problems. Call Kanstul and/or your sales rep and see if they will take care of an inspection under warranty. Could be a loose solder joint or brace. Or... there might be a solder blob inside the horn. Have you tried using 3 instead of 1 & 2?