Like planebow said, hit the palm of your hand into the grunt tube (mouth end). It will make a popping sound. I can't tell you that I have ever used it on elk though.
Closest I have gotten is by being able to hit your diaphragm with breath going in and out. It gives it depth that you can't get just hitting the end of your grunt tube, but it is tricky.
I glunk by clinking or popng my tongue and ad depth by using a grunt tube. This does not produce a lot of sound but neither do elk when the glunk. This is a awesome sound to use when you are in amoungst em and trying to close the deal on a herd bull.
Hitting the grunt tube or droping your tounge from the roof of your mouth( Imagine a big sloppy dog eating) Works good in Bedding areas to realy piss off a herd bull who thinks you are running one of his hot cows off!!!!