Ah, I think you've solved it for me. I mistakenly thought that the Durso actively siphoned the water down into the sump. I think what's happening is that the airhole in the end cap is getting partially plugged, resulting in a (very noisy) siphoning action that is slurping water out of the overflow chamber. I'll try cleaning out the vent hole, and we'll see if that takes care of it.

Thanks so much!
Derry