I've heard that too. But if you watch/listen to interviews with Ian Anderson, he is very intentional about when he does it. Whether you like it or not, it's his "trademark" on rock flute playing. I'm not crazy about it either (I find it weird) but I love his music anyway.A professional flute player told me his humming and grunting into the instrument was a trashy technique.
I would imagine any classically trained musician, seeing someone go completely off script on their instrument, would feel the same.