It is illegal to tie a sump pump into a sewer in almost every town. You can tie them into storm drains, but thats not all that common. Often times the sump pump is tied into a dry well (basically a large hole in the ground filled with rock that will allow the water to drain naturally back into the soil away from the house). Maybe this is what your setup is like.
Your sump pump probably has a backflow preventer and so long as it does the water the pump pushes out shouldn't be going back through the pipe and into the sump pit. You should verify this. If this checks out maybe you should do a little exploratory digging and see what the noise is. It could be just the last of the water draining out of the pipe and nothing to worry about.