Toilets, Sinks, Showers, Tubs and Disposals - Bad smell from new ensuite bathroom

Hi, we moved into a new house in February 2006. In the last week we have been getting a very bad smell from a small bathroom adjoining our bedroom. The bathroom contains a shower, a sink and a toilet.
When we pour water into the sink for 10 seconds and then turn it off, we hear a very hollow sound in the pipes as the water flows away. We haven't heard it before this week.
We have only used the shower in this bathroom once or twice since we moved in and it has been at least 3 or 4 months since it was last used, but the toilet and sink are used daily.
Would you have any idea why this smell has appeared so suddenly and what we can do to get rid of it?
Thanks.

It sounds as if you have a venting problem. I know the house is new, but it doesn't take squirrels or other small animals to drop nuts, leaves, twigs, etc in your vent pipe. Without the vent pipe properly allowing equalizing pressure from the atmosphere, water will not flow properly in the drains.
Place a straw in a soda and put your thumb on the end of the straw. Withdraw it and the liquid will stay in the straw until you introduce equalizing pressure from above by removing your thumb.
If you feel comfortable and safe doing it, go up on the roof and chug the vent pipe with a garden hose. You will feel the obstruction and it shouldn't take much to dislodge it.
Post back if we can help further. Good luck with it. Be careful, as falling is fun, it is the sudden stop that makes for a bad day.