How do I keep them from “burping”?
Well, you … no, too easy. 
Conventional wisdom is to ensure that the leather flap is hanging downward, i.e. that the hinge is at the top, and gravity will pull the flap downward and closed. Sometimes the leather flaap can dry out some, letting a bit of air in. A dab of leather softener should do the trick, the kind you’d use on a baseball mitt.
Just to confuse matters, I found that turning the valve in my bellows around so that the flap hinge is at the bottom solved the burping noises, but that’s just my experience.
djm
I glued a small washer on the back side of my blowpipe clack valve. The extra weight eliminated the rude noise.
Peace!
Reepicheep
Had to laugh, sorry… too many beans I guess 
it’s okay. I laughed when I posted it. The thread warrants laughs. 
more weight, less weight, stiffer, less stiff, any of these can sometimes fix it.
The problem results from a resonance i.e. a standing wave forming in the blowpipe. If you can change the natural frequency of the valve’s flapping so that it doesn’t reinforce the standing wave, the problem tends to go away.
Pat Sky published a method for fixing this in the Pipers’ Review a couple years back. One fix involves a bit of rubber cement dripped onto the thinned ‘hinge’ part of the leather, which makes it a bit harder to open and close.
Bill
A contribution from the lunatic/junkyard wars fringe:
Try a binky. The rubber nipple from a pacifier or baby bottle can be mounted on a tube, inserted into the blowpipe and cut in a star or flap(both work) opening. You need at least four cuts to make and eight point star. I’m using it and get little or no blowback. Less cuts get more vibration. Hope it works for you*.
Marc
- Note: The physical universe changes from workshop to workshop.
