I've always called it a growl. I do it by snorting, but further down in the throat (I'm not very good at it). Seems to be more popular in southern gospel and country than blues.
I wish I'd seen Terry perform in person. Didn't know he was going to leave so soon. ---------- Ozark Rich
It is a snore on the in breath, like you were clearing your throat. A growl, a snore call it what you like (cue quote from Sonny Boy Williamson ) but thats how to get it. Trumpet players do it in Dixieland Jazz. Another effect is rolling your rrrrrrrrrrrs but inwards, a little tricky but a good effect.
After ReListening and realizing it was draw,I am in total agreement with you guys. I can only get that sound with a draw and if it is a blow I will use a tongue flutter. I am not completely new @ harp,but I still have a ways to go.