There are a few of warning signs that your main sewer line is clogging. Pay attention and deal with the problem before the sewer clogs completely and backs up into the house. (1) Slow draining sinks and bathtubs. If you have two or more slow drains in the house, the problem is probably in the main sewer. (2) A gurgling sound when you flush the toilet (3) water comes up in the basement floor drain or a first-floor bathtub/shower when the toilet is flushed (4) There is a constant need to plunge toilets. (5) sewer smell coming from drains.