Here's my story on this topic (hope it helps):
A couple of weeks ago we noticed a small pool of water in our furnace room at the main drain. It had at some point during the day actually got under the walls of the nearest room and had soaked under the carpet nearest the wall of that room. Every time we turned on a tap or flushed the toilet the puddle would grow.
The puddle had obviously receded a bit by the time we had discovered it. At some point during the initial flood the sump pump in our furnace room must have come on and that served to contain most of the flooding, except for the nearest room. Without the pump I suspect the flooding would have been terrible as we had the washing machine and dishwasher going that day at around the same time.
Anyway, we called a plumber and explained the problem and he thought immediately that it was a main line sewer back up. The sewer main line is actually City property and thus it is their responsibility to look after it.
We called the City and they sent out a crew within an hour. The crew had to locate the end of the main line, which was actually more toward the front of the house and was not in our furnace room. They had to rip up some the carpet and some boarding to get to it, which kind of sucked. They then snaked some metal tubing down the line and managed to dislodge whatever was causing the stoppage.
Later next week another crew came out and checked the pipe with a cool little video camera. There appeared to be nothing wrong with the piping so it was more of a matter of something becoming lodged and jammed in the main line. The City crew actually recommended we use more water than we presently do sine the flush saver toilets we have in our house may be to blame (they use the minimum amount of water per flush which can apparently lead to back-ups).
No charge from the City for their activities (although we did have to hammer the floor boarding back down and relay the carpet, so it's probably even in that sense).
From the sounds of it, your problem could well be similar.
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