What makes this pattern urgent instead of something to monitor? The safest answer starts by preserving timing, location and trigger information rather than assigning a cause from one clue.
What this question is really testing
pressurized supply water, gravity drainage and vapor or gas pathways create different timing signatures. The observation becomes more useful when it can be compared with supply leak, drain connection leak, failed seal, blocked vent, dry trap and unrelated weather entry.
A useful next observation
Inspect accessible connections without opening concealed assemblies. Record the result together with conditions that stayed unchanged so the comparison remains honest.
Safety and handoff boundary
Stop testing if water approaches electricity, finishes sag, wastewater escapes or a pressurized leak cannot be isolated. Licensed plumbing work may be required by local rules. Share the exact fixture sequence, delay before symptoms, clean-water or wastewater indicators, accessible photos and shutoff behavior.
