I am hearing this running water/sloshing around sound coming from the vent stack, but it only happens for 30 seconds or so and maybe once a day or every other day, pretty irregular. The problem is none of my plumbing fixtures are draining any water into the waste line when this happens, so I don't know where this sound is coming from. My toilet isn't leaking either, what could it be? I'm hooked up to city sewer, no septic.