Welp, I'm an idiot
Finally got the plumbing sorted on my 60 gallon tank last night. Everything was running well, but the sump is splashing like crazy. I need to put a door on that cabinet. It was so loud while watching TV last night I had to turn off the pump. It's OK, since I tested to make sure that there was enough capacity in the sump to handle having the pump off.
However, I must have bumped the output of the pump so the tip was under water in the main tank. This created a siphon and when the pump was off it siphoned an extra 10 gallons into the sump. 10 gallons that the sump couldn't handle.
After the silence of the tank for a couple minutes, I heard splashing again. The house was dark because we were watching TV, and I stepped into a deep puddle of water in front of the tank. Slipped and fell on my butt. A bunch of water had overflowed the sump and pooled on the craptacular laminate flooring.
I could have kicked myself...
The original splashing noise is from the water dropping into the filter socks. With the cover on my sump and a door on the cabinet that noise will be gone.