The last eight months we have noticed a bad eggy/or mouldy odour getting worse from one of the cupboards in the kitchen. We think the water supply pipes for the washing machine are the problem inside the cupboard. The smell is coming from under the floor where the water supply pipes are. There are no obvious leaks having taken up a few floor tiles, but we are worried and at this stage dont want to rip up the entire floor. I have heard of natural gas build ups in the pipes?
Sometimes when the plus is pulled from one basin, or the washing machine or dishwasher discharges, the force of the water rushing through the pipe can leave a vacuum behind it. This can suck out water from the trap under the sink allowing foul smells to come out of the plug hole. If this cannot be identified as the cause it needs to be investigated I'm afraid which may mean a little mess.
I used to have this problem too. The cause of the smell in my situation was that the washing machine flexible waste pipe looped down below the level of the waste water trap under the kitchen sink which created back suction (called siphoning I believe), so the water in the water trap was sucked out by the washing machine pump which meant that the water trap was empty and the smell from the sewage waste pipe drifted up out of the kitchen sink drain, phew! It was simple to remedy by re-routing the flexible washing machine waste pipe so that it no longer looped below the level of the water trap. Colin