This started to happen a couple of months ago...It is possible that I did a bit too much on a day I was preparing for an audition...
Since then double buzz appears now and then mainly in the middle register and in dynamics louder than mf.
First I thought it was just fatigue, tired embouchure muscles or something of the kind, but since it appears in different situations, sometimes when I am tired and sometimes in the beginning of my daily practice I haven't found a pattern yet.
I know that at the end of the day I will have to figure it out on my own, but suggestions may be helpful...or not
P.S. Probably I should mention that I have no particular problems with range or endurance.