Air gets trapped in the drain, and "gurggles". What kind of drain is on the overflow? Generally what happens is that a bubble of air gets trapped inside your drain and it backs up water until it "burps". You might try putting a durso in. I've also found that running a piece of tubing - a little bigger than the "airline" tubing, the hard white tubing from HD - down into the drains helps keep them "burped", even improving the performance of a durso.
Bill
215g FOWLR... and anemones, GSP, gorgonians... carp, that isn't FO!
"I killed my first SW Fish in 1971..."