For the past five flushes, when I flush my toilet, I hear noises coming from my bathtup drain. The bathtup is next to the toilet. It sounds like water is being drained from the bathtub pipe. What would cause this?

Here, check this one out:
https://www.askmehelpdesk.com/plumbing/tub-drain-gurgles-when-toilet-flushed-6821.html