This may sound silly, but I've noticed that when I use vibrato, I tend to make sounds as I play it :/. I think it's coming from my throat, because I can feel a slight vibration in my throat. I also hear sound, almost like a humming sound when I play vibrato. The sound is not exactly the note that I'm playing, but if it's a higher or lower note, I hear low or humming. I don't know if this is due to to0 much air when using vibrato or what, but I don't remember have this "problem" when playing before. It's has just recently started, about 3 months, so I don't know. I'm not sure if anyone can relate or not, but I do find it quite strange.

your throat should be very relaxed when you play, the sounds you are describing sound like they are due to tension, which can produce a fast out of control vibrato as well as throaty sounds...

I think I have a similar problem.. I just started doing it a few months ago (when I got my new flute?) and its only when I do vibrato.

I've tried to think about it and keep my throat more open while I'm playing.. it does not help. The only thing that helps is to not use vibrato.. thats not really an option :P

My teacher has never said anything to me so I guess its not that big of an issue? I'm pretty sure you cannot hear it unless your are RIGHT next to me while I'm playing..