A partial blockage of the sewer pipe could cause this. A partial
blockage from roots could come and go if something occasionally gets
hung up on the roots.
Is there anything else happening in the house when this occurs, such
as a toilet in an above floor being flushed or a washing machine or
dishwasher draining? Even a shower could cause this to happen if the
water drains too fast.
If there is a partial blockage, the slug of water rushing through the
pipe from another toilet flushing or washer draining will hit the
blockage, bounce back and push air back into the system. This air will
find the easiest way out and that could be the gurgling toilet. You
don't get any water backed up because the blockage isn't big enough to
fill the drain.
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