by Why do I hear gurgling in the shower when the toilet flushes?
Short Answer: Air pressure changes caused by restricted drainage or venting are likely occurring.
Detailed Answer: When wastewater movement is impeded, air may be displaced through nearby fixtures. Common causes include partial sewer blockages, vent obstructions, septic restrictions, and drain field problems.
Related Questions: What causes plumbing gurgling? Can vent problems mimic septic failure? Why do pipes bubble during drainage?