At the end of a flushing the water in the bowl stands much lower than hitherto: at the level of the top of the exit pipe, raising the possiblity that the S-bend will not fulfil its function. There is also a gurgling sound at the end of the flushing process.

Norman Weatherall, July 2007

if this has just started and operation ok before sounds like the start of a blockage/restriction in drain pipework causing water to fill drainage pipework completely when draining away thus forming a vacuum in pipework which sucks water out of pan plung,rod or jet through drain as required

tony,tsp heating, July 2007