Go to the roof vent. (Safety first: don't do this on a wet or icy roof). Shine a flashlight down the hole. If you see a solid wall of black sludge or a nest, you found your problem.
Most vent clogs are "saddle clogs." They sit at the bottom of the vent stack, right where it turns horizontal to join the main sewer line. Water never washes that area. Waste solids and grease sneak up over time, creating a hard, calcified shelf. drain vent clogged
Your plumbing isn't broken. It’s just holding its breath. Give it air, and watch the water fly. Go to the roof vent
It stops leaves, birds, and squirrels. It stops the lint. It stops the mystery. creating a hard