On Sep 1, 3:04*am, ben wrote:
thanks all.

to answer some questions:

- it is not a well-sealed house. *built in 1953. *lots of drafts.

- nothing else is happening when the bubbles occur. *no other devices
connected to water or sewer operating.

- this toilet is in an original toilet location in the house and is on
the first floor.

- admittedly i can't see all the way down the vent. *perhaps i should
try better with a good flashlight. *there's certainly nothing within
the top 5-10'.

"admittedly i can't see all the way down the vent. "

I'm not doubting this as a cause, but I'd like an explanation as to
how a clogged vent would cause this symptom.

How would a clogged vent force air back up through a toilet?