the water is hitting the sump level and causing noise. there are several ways to quiet this.
one is to put a 45 degree elbow about a foot above the water level in the sump. run a pipe from the elbow to water level in the sump. put a Tee there. it works best if the straight part of the tee is one size larger than the tee part. the tee part connects to the pipe coming from the elbow. now run a pipe down from the tee into the sump water. a good 6 inches or so. on the other end of the Tee run a short length of pipe up as a chimney. put a cap on this with a tiny hole in it like the durso. i have a drawing of it around here somewhere.
this works wonders for my wifes tank.
G~