A grunt tube is good, but Primo's can is the ticket! I have literally had bucks run into my stand when I use the thing. It is an estrus bleat, and I have had a few ocassions where bucks will actually trot over to check it out.
As for a grunt tube, I use Primo's little one that is adjustable. Not sure what it's called, but it makes 5 different grunts from an estrus bleat all the way to a mature buck grunt.
"Because a duck is just a 3 pound critter"