More good news/bad news unfortunately. I cut the pump out and added an un-welded pvc coupler so it'll be much easier to get at the pump in the future. I also built and added a standpipe to the overflow box, which works like a dream. The drain is completely silent now.
Bad news is that the pump, which wasn't all that
quiet to begin with, is now super noisy, even after cleaning it out. It also doesn't start up immediately when I plug it in, you kind of have to knock it against the side to get it going. Sooooo... looks like I'm getting a new pump. I'll probably still come out ahead dollar-wise compared to buying a canister, but this is a pain in the butt.