: Every so often my toilet will let an air bubble. No septic come up. It has only happened 2 times in the last couple days and noticed tub drained slow. Unplugged all drains and cleaned..could septic tank be bad?its possible that the tank is backing up on occasion and blocking the vent. not knowing the elevation of plumbing system in relation to tank , its hard to say for sure. are you seeing this bubble in the bowl or just hearing it. sometimes the ballcock in the toliet tank will let out a bubble as it finnishs refilling because of worn out parts.