edit
You can growl and play at the same time too! …probably more popular with sax players tho.
I’ve spent 5 minutes trying to play and hum at the same time. Unsuccessfully. 
Maybe it’s just something you’re born able to do, like curling your tongue. 
edit
Hm, that makes more sense! I can kinda do that, but then I can’t get a good sound. If I blow hard enough to get a good sound I can’t keep humming. It’s a talent that must be perfected I suppose.
Cranberry,
This has potential. The sound I get is like a completely new instrument, kind of a cross between Ian Anderson, a kazoo and a whistle. But certainly not Irish.
How long will it take me to play and hum “Row Row Row Your Boat” as a round? You could do harmonies, counterpoint. My goodness.
Best wishes,
Jerry
Singing/humming when playing the flute was popular among jazz artists beginning in the late 50’s or early 60’s. Rahsaan Roland Kirk and Yusef Lateef were among the best – Ian Anderson even did a little bit on the first few Jethro Tull albums. It’s a really great effect when done well.
PS – Rahsaan was also able to play THREE saxes at once. Needless to say, he was a master of circular breathing.
Ahh yes, Bretton, the old growl and play at the same time. It makes for quite a sleazy saxophone sound.
Some harmonica players also use humming as an embellishment.
Roland Kirk,Yusef Lateef..Great exponents of vocalising whilst playing the flute.Good point about Harmonica players,C Don-Sonny Terry used to hum,squeel,yodel etc. to great effect along with his ‘Harp’ playing.Listen to some of these players if you can Cranberry,they are bound to give you further ideas in your ‘sonic explorations’!
I’ve been told that I grunt and moan and groan when I’m drumming, and since then I’ve caught myself at it a few times. It takes all the concentration I can muster to play drums, so the oral sound effects are completely unconscious. How some folks drum and sing at the same time I’ll never know.
I have been caught dancing to my own whistling…
I remember one of the many times I saw Phil Cunningham he did sort of a goof on bagpipes by humming a D drone while he played his whistle. It was pretty funny. I gave it a shot later at home. Not as easy as it may seem.
Cheers,
David
My 2 cents…
Humming,squealing and other vocalizations while playing is a basic technique of the Digeridhu. Self accompaniment was the norm for most of our ancestors and their instruments for hundreds of years, an example,The Arghul, with its single reed drone and somtimes a whistle chanter,the zummara(see avatar), as well as a great many double/tripple “Air reed” or “Duct flutes” throughout history.
I am a personal advocate and promoter of quiet parlor woodwinds that can be played behind soft vocals and “fill in” behind melody playing at a low volume. It is a very simple thing to make a whistle play sweetly and quietly by simply making the voicing half size. I also think that adding a single “free reed” drone for “Dord Fian” (Traditional Irish pedal point) is not without merit for the solitary apartment player. I just don’t understand why these lovely instruments are not made available by instrument makers. 
Arghul photo
http://www.bbc.co.uk/aboutmusic/features/africa/images/ninsarghul.jpg
Launeddas photo
http://www.launeddas.info/images/foto1.jpg
Some very interesting points raised there,Thomas.Actually,I had a bit of a laugh the other day-I was telling someone at work that I was getting a Practice set of U.P.'s built,and that the usual ‘way up’ was to start with the bag,bellows and chanter,then add the drones(and finally,regulators).This person,asked me how much drones cost,and when I gave them a rough estimate,their reply was “Why don’t you pay someone to play accompaniment?- it would be cheaper!”
Glenn Gould did it all the time, some of his recordings even captured it and they weren’t removed. Eirie to hear them when you weren’t expecting it.
MarkB
Gould’s singing was described as that of a ‘stoned bee.’
It was moving, cause here was this guy
with dizzying technique, I expect the greatest
interpreter of Bach ever–playing perfectly,
absolutely in control and…singing!How happy he must have been!
I think vocalizing on the flute may help
playing blues… Must try it.
How many pairs of hands did he have?
And where did he shove the flute?- No the wonder he hummed when he played!! 
