I wondered if anyone can explain a strange thing that has been happening to me recently. My house is 14 years old. The guest bath is on the second floor directly above the first floor half bath and laundry room. Very recently (within the last 6 weeks or so), when the guest bath tub drains, air bubbles up through the downstairs toilet bowl, then the water in the toilet bowl drains until it sucks air, then it stops. Last week, some soapy water bubbled up into the toilet bowl as the tub was draining. The only other plumbing problem I've had with any of these fixtures is that the sink drain in the guest bath was pretty well clogged late last year, but I snaked that and it has been fine since, although when the sink drains you can hear it through the tub drain rather loudly. Any idea what might be causing this? My only thought is that the sink clog may have been so bad as to restrict the vent line, so now the connected drains are getting their air break wherever they can find it.

Thanks!

You have a partially clogged line which is why it bubbles into the toilet. Do you use the tub, because it sounds like its trap has gone dry which is why you can hear drainage in it.

Thanks for the info. Yes, the tub gets used daily. Usually just for showers, but probably once a week or so for baths. I can't imagine the trap going dry with that kind of usage.

What would you suggest for the partial clog? Just a plumber to do a better job snaking it out than what I can do myself?