When I don’t keep pressure in the bag, my bellows sound like I haven’t fed them in days. The noise is coming from my bellows inlet flapper and gets very annoying particularly when I’m sitting in a TINól class and everyone thinks I forgot to eat breakfast. Of course, at a tionól I do sometimes forget to eat breakfast, at least the solid kind, but that’s not the point. The hinge on the leather valve is positioned at the top when in the playing position, and small changes in this position have no effect on the growling. So what’s the cure? Do I need to start eating oatmeal, or is there a tastier solution?
Try gluing a small washer or a penny to the valve flap. This should will make the flap heavier and should make the noise go away.
Although it could make it much LOUDER too!! 
Check that leatherflapper is straigth , also u can put abit weigth on it (qlue extrapiece of leather etc)on,it keep flapper correct place better and prevent leather bend.
Reduce the hinge stiffness so that the valve’s relaxed state is not in the open position.
I like this valve myself, works well: http://dennishavlena.com/ck-valve.htm
eat some mentos washed down with fizzy cola ,that should do it .
RORY
Thanks guys for the advice. The flapper was indeed resting in the open position and the edge was curled. A dime seems to have done the trick of filling it’s hungry belly. I’ll try the silicone inverted flapper if it starts leaking again. I also believe that my practice of storing the bellows with the inlet valve facing upward (flapper open) has contributed to the problem. I now store the bellows so that the flapper lies closed. Oh, and I did try the mentos, but that only got me a visit from the local animal control officer…something about a report of a rabbies outbreak!