My bathtub gurgles when I flush the toilet. Venting problem?
by
Bob
Suggested Project Book
This site might help. https://www.tradewindsimports.com/blog/troubleshooting-gurgling-drains/
I would say yes, it sounds like the toilet is pulling the trap on the tube. Toilets are usually vented directly to a 3 inch pipe through the roof. Check to see if something (squirrel or bird) has built a nest. The other possibility is that the toilet was only vented on the sink vent pipe and it is blocked.
You might also use a root-rooter device to snake out the pipes. It may be a blockage in the pipes on the way to the sewer line in the street.