I'm hoping someone can explain this behavior. About once a day, our toilet will suddenly just drain out. The cistern remains filled, the toilet doesn't actually flush, it just drains down until it gurgles; shortly afterwards, the tub and the sinks will begin to gurgle too. The noise from the sinks and tub will stop as soon as a little water is run down them. The toilet will continue to drain for a short while, though - it usually takes several flushes (anywhere from three to eight or nine) before it stops and the water level settles. After that, it's normal again for the rest of the day, and you'd never know anything weird had happened at all. There's no smell accompanying any of this, either. Also, while it's still slow draining, the water in the toilet rocks a lot; back and forth, like a tide machine, instead of just level. Once the slow draining stops, the water levels again. No idea if that's important, figured I'd mention it in case.