What the vent and the trap do
Every fixture has a trap that holds a small water seal. A King County plumbing installation page explains that the trap, when properly vented, keeps that seal in place so sewer gas can't come up into the house. Vent piping lets air in and out of the drain system so the seal isn't pushed or pulled by pressure changes.
The International Code Council's commentary on the plumbing code's vent chapter describes what happens when that air balance is off. Higher pressure on the drain side can push the seal back toward the fixture, and it gives bubbles rising through a toilet's trap seal as an example of that. Lower pressure can pull water out of the trap and leave a partial or empty seal. That is why a toilet that burbles or loses water when a tub or washer drains is worth having checked. It doesn't prove which part of the system is at fault.
A drain restriction is a different possibility
A blockage affects the path wastewater takes out of the house. The Philadelphia Water Department's flooding guide says clogged or cracked laterals, the sewer pipes that connect a home to the city sewer, keep wastewater from flowing out of the home properly. It names wipes, grease and cooking oil as things that can clog a lateral and cause sewer backups. The guide also says homeowners are responsible for their laterals. So it helps to ask whether a restriction could be in a shared fixture drain, the main house drain or the lateral, before anyone picks a repair.
Write down the pattern
Before a plumber visits, note which fixture sets off the sound, such as a tub, shower, sink or washing machine. Note whether the toilet water bubbles, rises or drops, and whether other drains are slow. Also note when it started, whether it followed plumbing work, and whether it gets worse in heavy rain. The PWD guide says basement toilets, sinks and drains are vulnerable to backups when heavy rain fills the city sewer. A short recording during ordinary use can help, but don't flush over and over or dump large amounts of water just to make it happen.
Our guide on documenting basement water problems and our list of documents to keep after a repair cover what's worth saving.
When to call a plumber
Arrange an inspection if the gurgling keeps happening, especially if other drains are slow, the water level in the bowl changes on its own, or you smell sewage. Ask what findings separate a vent problem from a drain restriction, which part of the system needs a closer look, and what evidence supports the repair they propose. Leave roof work, vent changes and drain-opening work to qualified professionals. If your home uses a septic system instead of a public sewer, tell the plumber. Our contractor hiring checklist and our guide to which Philadelphia plumbing jobs need a permit can help when you get a quote.
An actual backup is more urgent
Wastewater coming back up through a toilet, shower or floor drain is more than a noise. Stop running water and flushing, keep people and pets away from it, and get professional help quickly. The PWD guide says that if your home has frequent basement backups, you should call PWD before pumping water out, so it can schedule an inspection and sample the water.