if you have a basic 4" water trap then it will pull air through (and make a gurgling sound) when it sees a pressure differential of about -0.15psi, i.e. not much. I would imagine that this is well within the scope of Bernoulli-induced pressures whilst there is a flow in the soil pipe.
If the soil pipe is 'filled', even briefly, above the connection to the basin, and there is no suitable vent, pressures in the pipe could become positive, in which case the trap will gurgle and burp stinky gas into you home. Not nice!
Most folk put up with the first gurgle but not the second; it all depends where the vent is I suppose.
cheers