I too have wondered about this and consider the following explanation likely:
We know that flow in a rigid pipe exhibits the Bernoulli effect when it encounters a constriction: the flow speeds up and the static pressure measured right next to the inside wall of the pipe goes down.
But the pipe in question is not rigid which means the reduced pressure exerted on the walls of the pipe by the flowing fluid will have a slight tendency to suck in the walls and make the constriction worse.
At some point the flexibility of the walls will allow the constricted point to move in the direction of the flow towards the pipe's outlet, assisted by the pressure buildup behind the constriction (which tends to make the pipe bulge out there). The pinch point then propagates as a wave down the pipe until it hits the end and transiently modulates the flow there- and the tip of the flexible pipe goes "blurp".
There are roadside advertising devices that look like huge tubular mannikins that "dance" when air is blown through them; in this case the constrictions cause the "arms" of the tube to slide towards the "hands" and in so doing make the "arms" whip around in a dancelike manner.
This propagation of blurp noises is the source of the balloon "squeal" and the explosive splatter noise produced when you sit on a "whoopie cushion".