Originally Posted by
speedball1
Hi 89 and welcome to The Plumbing Page at AskMeHelpDesk.com
I have a different take then Milo. Milo thinks there might not be a tub trap while I think that's where the sounds coming from. Let me explain.
Vents are there to relieve suction caused by draining water, If a vent's blocked the suction has to relieve itself some where, In your case the tub trap. The sound you hear is the suction being pulled through the water in the trap. Your solution would be to snake and clear the bathroom vent. Good luck, Tom