U have minor blockage in waste line.

Three ways to try to correct it.

Best way would be to snake down from roof vent. Run out about 15 feet of snake past toilet.

Next best= Remove tub overflow cover and stopping system. Run snake through overflow.

Third is more of a preventive method. Pour 2 cups of vinegar and quart of warm water in tub drain before going to bed. Let sit overnight.

In morning flush with 2 gallons boiling hot water.

After drain is clear and working correctly, treat all drains in house once a month.

This is done with 1 cup of vinegar and 3 cups of warm water at night and flushed with gallon of boiling hot water in morning.

Chuck