Hellow everybody.. I hope this post is in the right place.. If It is not, I apologize

I have been playing a new instrument for 6 Months.. It is a Schagerl (wich is an Austrian brand) 450g (a student model).. My teacher told me that, the brand was very good (he uses a kissbone wich is a professional Schagerl bone), so I bought the trombone.. any way.. I have been playing it for 6 months with a 51D schike mouthpiece and the sound was dark, very deep, rounded and everything had been perfect.. until the last week, When I was at the middle of a class and something started to sound inside the trombone, like a metalic buzz.. It was very low.. but I could hear it.. and of course It was annoying.. my teacher told me that may be the problem was in the rotor sistem (something was loose)..
the next day I tried to play at home and the buzz was still there, but now that I was alone, I could realize that there was something else: the sound produced by the trombone wasn't the same.. the produced notes were tuned but now the sound wasn't too deep, it was more brilliant, it was lighter than before and It was more metalic.. even with the schilke 51D.. and when I tried to play loud the sound got saturated very easy..
Becouse of this problem, I took my bone to the local "luthier".. and he told me that maybe the rotor sistem was loose.. so he fixed it... and yes.. the buzz was gone.. but the brilliant sound was still there.. he is not a musician so he couldn't hear those sound characteristics.. even like that, he made a fast checking of all the welding places to se if there was something broken.. but everything seems to be ok..
After this, I took my bone again to my teacher.. he checked the bone, everything was in its place and everything seems to be adjusted, He played the bone.. and he told me that the bone was fine.. the sound was ok.. he was trying to hear if there was something wrong with the sound but he couldn't find it..
But of course Im the owner of this trombone, Im the guy that plays it every day.. and I am accustomed to the way my bone produces sound.. and yes.. the sound has changed.. Im very shure of this.. yea, the notes are tuned, the sound It produces is ok (actually, It sounds like a small shank trombone) but thats not the sound It used to have (heavy, dark, deep) like a big trombone..
So now I really need all your oppinion and all your experience to try to find whats the real problem or the real cause of this...
I really will appreciate it.

Can you post sound clips of your playing?