About a month ago a bad smell started coming from our venting inside our home. It smelled like a big toilet. We had someone come look inside our venting and nothing. We had someone come and check our furnace and nothing. And It doesn't smell everyday either.. right now it smells terrible but it is the first time in over a week. We had a fix it guy come and remove the spring vents in our attic, their were five of them, he tied them all into the septic vent through the roof. We thought that would fix the problem but the smell is back and it is worse! Please help! We are at our end. We have a propane furnace and we are on a septic system. There is also an apartment downstairs with a bathroom.