gurgling toliets

Occasional both of my toliets gurgle when I let water drain out of either bathtub. This has been going on a couple of months and is getting worse. I have checked the white pipe (clean out) coming out of the ground and the water is flowing fine. The house is two years old and has a septic tank. The septic tank failed last year and another 100 feet was added to the field lines.

Yesterday I checked the traps and pipes under the kitchen sink and one bath room. They both (all pipes & up in the pipe connected to the house) was covered in this black slimy stuff which I cleaned off. Is this normal? I saved a 2 inch piece of this stuff. When it dried it almost felt plastic. What is this? I have not checked the other two sinks. Any ideas of what else to check would be great.

Thanks

It sounds like the vent stack is plugged.
DIY or have somebody else go up on your roof and look into your vent w/a flashlight. If there's something in there, try a wire clothes hangar if it's close enough to the top. If it looks like it's big/solid enough to plug your sewer line if pushed down into your stack, call a drain cleaning company. They can put a snake down it and pull it out.

As for the black crud, it's actually build-up from normal use. I clean it out of customer's pipes all the time. It's soap/grease/human grease build-up. Nasty as it is, using drain cleaners with acid in them to get rid of it will damage metal pipes. Look in your local hardware store for a drain cleaner with bacteria/enzymes. They eat the nasty stuff and won't harm your pipes. Some drain cleaning companies carry it - it's a "drain maintainer" or for preventive maintenance. If you go that route, ask for a free sample or a demo before buying.

"And away go troubles down the drain." tm