Why does one drain gurgle when another fixture empties?

The useful answer starts with timing and pathway

Shared drainage changes pressure across traps. The triggering fixture often matters more than the fixture making the sound.

Best first comparison

Document when the symptom begins, what condition preceded it and where the first visible or measurable change occurs.

Before deciding on a repair

Compare at least one credible alternative cause and choose a check that can separate the two without creating new damage.