Why can the gurgling sound of water only be heard when it's disappeared down a drain, and not before?
This happens on direct vented fixtures that have no actual vent; they're close enough to the stack (&lt;5') not to need one.

And when there's a 'head' of water above the trap while the water's running, there's too much suction to let any air in, so it pulls air down the stack. Once it has less 'head' it's easier to pull air past the water in the trap then it is to pull it down the stack. : gurgle gurgle

Basically, when water travels down a vented pipe, it creates an air pressure differential. The make-up air will take the path of least resistance. Be that, pushing a little bit of water out of the trap to make an air gap, or pulling it all the way back down from the roof.