I'll chime in here as well. This has happened to me the last two rehearsals my band has had for its THIRD of July concert: my horn sounds like it is being run through a fuzz box (at high volume). Since I am using a Conn 30I, my problem cannot be traced to "too much horn"; I think it is due to worn out chops, since the buzz only presents at the very end of the program when my lips are pretty much played out. As Dave suggests, it's a conditioning thing as much as anything else.
Harry Nuttall
Bach Stradivarius New York model 8II tenor trombone #28xx
Besson New Standard #438xxx
Besson "Prototype" euphonium #510xx
Conn 30I Wonderphone double-belled euphonium #327xxx
Hawkes & Son Excelsior Sonorous #534xx
Holton Revelation euphonium #753xx
Holton Revelation euphonium #797xx