I read somewhere something on this and it was put down to the rubber seal in the outlet pipe the toilet pan outlet pipe fits in to. It apparently sucks in air when toilet flush causing a noise similar to passing wind from the rear end or in short a 'Fart'. The cure was said to be silicone bead around and against the rubber seal to prevent any air getting in. It's a thought and have never had the oportunity to try it.
Bob, May 2009
