It could be the drain(s) building up with sponges and other things or your return pump has changed it's power output ever so slightly (or getting dirty)? Easy fix is to adjust your gate valve until it clears out the bubbles. Happens to mine from time to time, usually means I need to clean my return pump or my return lines. There's so much life in the dark areas (like return lines, drains, etc) and it doesn't take much time for it to build up.
I'd be surprised if sponges and whatnot were in there- my tank hasn't been running that long. I'm still in the ugly new tank diatom phase. But tomorrow morning when the return shuts off for breakfast, I'll climb back there, pull that standpipe (it's just friction-fitted into the bottom bulkhead), and give it a good cleaning, just in case. Heck, maybe it's just loose in the box and extra water is creeping down the gap and down the line.
It's kind of hard to tell what's going on from the video... But here is something you can look at.
Look at the water flow from the top and see how much is entering the secondary drain. It should only be a trickle, not enough to cause burping or surging.
If the flow looks low from above (like only enough to trickle into the pipe), I would suspect there is something in there, like a snail or something. The blockage could be causing water to backup until is becomes enough to clear itself - and you get a surge or burp.
If you open the valve more it will slow the flow in the secondary drain and probably stop the burp, but if there were no other changes you should make sure the pipe is clear.
HTH
I'm kind of an idiot, and I didn't make it very easy to get to the exterior box. I'm too, ah, well insulated to squeeze between the tank and the wall, and I built the stand 3' tall, longer than the tank by 2', with a 2' tall tank, so the top of the overflow box is not only three feet away out of reach, but eye-level. I
can get a step stool, and carefully balance on the end ledge of the stand, but it doesn't afford a great view because I have nothing to grab onto. That being said, the box is pretty cramped, so I can't actually
see how much water is going down the 2° drain. It's a down turned standpipe that looms directly over the 1° drain, which is just a pvc strainer sitting in the bulkhead (with a gate valve further down). If I
was able to get just a dribble down the secondary drain to start a decent, silent siphon, would that mean both drains would have roughly equal flow as the siphon pulls on the water? Or will the primary drain still be carrying the majority of the load?
Has the water level risen around the sump end of the secondary? With such low flow I’d dump the sock.
Um, funny story about that. It's embarrassing to admit, but I chose quiet flow over even flow. I never did manage to get both the primary and secondary drains moving roughly equal amounts of water (which now I'm beginning to doubt is even the goal of a bean animal overflow). Every time I tried to get a siphon started on the secondary drain by using the gate valve on the primary drain- aiming for that small trickle that turns into a silent siphon- I'd essentially be working a toilet: water level rises, reaches a breaking point, and then flushes noisily on down. So most of my water flow comes down the primary drain. When I change filter socks, I pull the primary one out, move the secondary one over, and put a fresh one on the secondary. :/ I could totally dump the sock on the secondary drain if I leave the flow as is, but if I could ever manage to get a decent siphon going on the secondary, I'd have roughly equal flow going through both drains, right?
Thanks for your responses, guys! I really appreciate the advice!
