I own a rental property that was built in the 30's and I'm pretty sure it had a septic system at one time, I purchased it about 5 years ago and I was told it was in on the city sewer system..
Fast forward to the 17th my tenant called me an said that during the spin cycle of her washing machine all of her drains were gurgling, and finally water began backing up out of an abandoned sink drain(I didn't properly seal)
The house is a rather small ranch(1200ft) and only has 4 points that drain into the main(cast-iron). I had hoped it was a simple hairball, or frozen pipe. I pulled the toilet and attempted confirmed that each drain in fact ran into the main(toilet is right on top of the main) and discovered that all drains into the main do run without hesitation.
I called a relative that has some knowledge of plumbing both new and old and to him it sounded like I had a septic tank issue... huh?! I have city sewer! He said that it is possible that someone may jerry-rigged it into the city sewer via the septic tank and not the correct way by abandoning the Septic tank and tieing in correctly. Is that possible...legal??
I did a bunch of research since then and all of my research points to a sluggish, or clogged septic tank....WTF?? This is a exerpt from the folling website(http://www.inspect-ny.com/plumbing/DrainNoises.htm)
If the outside sewer line is partially blocked, or if waste piping to a septic tank or from a septic tank to a drainfield is partially blocked, the building drains may appear to work normally until there is a surge of usage such as an increase in occupants or when using a washing machine.
In lighter usage the waste and wastewater flow down into the main drain line or sewer line where they are in effect, "stored" while the waste slowly seeps past the partial blockage. As wastewater seeps past the partial main drain blockage a gurgling sound may be produced at fixtures in the building as air is drawn intermittently into the drains - an effect more pronounced if the building drain vent system is inadequate.
In heavier usage of building fixtures, such as when there are many occupants or when doing laundry, the additional volume of water may first cause this "gurgling drain" symptom to be more pronounced, and as the blockage worsens, the building drains may actually back up during heavy use. This condition can also produce sewage smells or sewer gas backups into a building."
The line that exits the house goes about 3.5ft down, turns to run parallell with the street, and goes out about 9' until it hits something(snake) in the interim I haver dumped a 1/2 gallon of Robeck septic tank cleaner in hopes that it frees up the clog..etc until it warms up here in MI and I can have a look to see if in fact I can find the tank..
you might have tree roots clogging your main line, their sould be clean out that you can run a sewer snake tru to cut the roots. some city's are helpful and some are not but you could call them.