Why does my sump pump pit smell?

I have a sump pump pit in my basement. There is a large inlet into the pit that seems to be coming from the outside area, probably drain tile? Someone installed a car port and new concrete in back of the house and put in 2 drains that run into the pump. The pump pumps into a pipe upward that goes directly into the main drain sewer pipe. There is a trap in this pipe. There is allways a stinky smell coming from the pit. It appears the smell comes from the big inlet from the outside. When the pump evacuates the pit, the inlet whole is exposed above the water level. Is this a problem? Should that water be over the inlet hole? Any help please.