Loud suction when draining the bathtub

Just did an inspection on a remodeled house where the bathtub makes a loud suction sound when it is being drained. No way to see if there is a ‘s’ trap. Bathtub is upstairs. Roof above bath has a 4 inch vent that I would assume the tub is vented to. My concern is potential siphoning from fast venting. Any ideas what else would be causing this? Thanks!


Role #1… NEVER ASSUME !!!

Actually, the few experiences I have had with a similar ‘noise’ was NO venting. The noise was from the drain ‘sucking’ air through the drain.