I have a house with underground basement. In the basement, there is washer machine and the washer machine dumps water into a laundry sink, under the laundry sink, there is a pump to pump the water up to a S trap (this trap is open, the pipe from the pump to this trap is not closed), this trap is connected to main sewage line of the house. The main sewage line is not going into basement floor but coming out of the house at about 4 feet above the floor. I smells foul odor when steps down the stairs connecting the main floor to the . I guess that when the water running into the S trap, some gas from the sewage line coming out and make the basement smell.
Is that right ?

Thank You for your answer.

It could be you sink too