Have you had plumbing work done lately? Make sure all of your fixtures have traps to block the sewer gas. Check the floor drain and make sure you see water in it. If the house is fueled by propane, make sure there is not a gas leak. Lastly make sure your roof vents are not blocked or if plumbing has been done not to code there may be a situation where a trap is being siphoned dry when other fixtures are draining.
I had a similar problem, however it has recently stopped. At first we could hear the air sound when we flushed the toilet and almost as suddenly as it started it stopped. Good luck.