Toilet blowback?

I'm not a plumber but it sounds like you have a situation with your drains whereby 'stuff' can flow out of the bowl and down the drain - but not freely into the main sewer - hence the blowback. If this situation has developed recently I would suspect a partly blocked drain between your house and where it joins the sewer in the road. If you can lift any manhole covers you may learn something. I once found a complete house brick causing a blockage to a drain for six houses (ours was one!)

Richard
January 2009