Well done. Interesting way to mike the hum with the sax in the background. For those who have difficulty with humming and playing at the same time, it have found it useful to have them hum and blow an airstream without the sax at first. It makes a "V V V V V . . . ." sound. Once they can do that "growling" is a piece of cake.
Thanks for the audio guide, very nicely put together!
I struggle with humming and playing a the same time and find myself following the pitch of the note I'm playing as well, which doesn't give the best effect, definitely something I need to work on!
A little technique I've doing something similar to a gurgle. It gives an aggressive growl effect almost like flutter tongue but also lands you with a sore throat, so not highly recommended!