My theory is that it happens at the "Y" junction. A bit like when the waste pipe of the bathroom sink meets that from the bath, when one is being drained the other often gurgles. This is due to liquid going up into the opposite pipe causing the noise. This might explain why it is noisier for some when lying down and others when standing. It all depends on how the individual is "plumbed". Mind you, some sleevers and banders complain of noisy guts too...
xx