We had a faulty cap in the basement on the sewer clean-out (there are no threads on the pipe to screw the cap into, so really the fault is the end of the clean-out, I guess). We had heard the rattling down there when the 2nd floor toilet flushed, but never thought much about it, until we discovered...matter in the basement around the clean-out that ought to have been flushed.
I bought a brand new cap that has a rubber gasket and wingnut that holds the sucker in (and keeps the "matter" where it should be). Now, when we flush the 2nd floor toilet, the 1st floor toilet belches (the 1st floor bathroom is directly below the 2nd floor). It's unpleasant if you happen to be on the downstairs toilet when this happens, but not horrible.
I now know that the faulty cap served a purpose in letting air escape.
So...do I just have a lousy vent or is this indicative of a larger sewer-line problem? Is this something I could fix on my own, or should I call in a professional?