Plumbing question...
See Pic. 1,2,3,4, and 5 are vent pipes for 1: left side of jack and jill 2: tub in jack in jill 3: laundry under jack and jill 4. right side of jack and jill 5. half bath under jack and jill. We are getting bubbles in the half bath toilet when the tub drains in the jack and jill. Smaller bubble sounds when a toilet flushes. Nothing from the master or the kicthen causes any issues. I just crawled on the roof and I looked down 1, 2, 4, and 5 and far as I could and couldn't see obstructions. 3 has a turn right under the roof. The drains should connection to the main drain pipe be in the following order starting at the back of the house heading to city sewer drain line: 3, then (1,2,4 joined upstairs and dropped down wet wall), then 5.

My guess is that the vent pipe for the half bath is clogged and when the water from the upstairs hits that zone, it's forcing the sewer gas out of the toilet?

Thoughts?

I have heard to drop a pebble down the vent pipe and have someone down below listening if they hear it hit the bottom.

I would be inclined to get one of those cheap snakes from harbor freight for about $15 and fish it down there. That way you can feel if there are any obstructions. And if there is an obstruction you can see how much force it takes to clear it.

I am no plumber but what you are describing makes sense.

depending on the vent your nose knows...