Large bubbles from return line when pump running above certain speed

[Diznaster]

My return line which runs up the inside of the overflow elbows into a lock line Y and a couple short sections of lock line (standard Aqueon Mega-Flo). It spits out large bubbles in oscillating pattern if the variable speed pump is over a certain percentage. Its about every second it spits out a few large bubbles. I have a couple ideas, but maybe someone has had this before and solved it. It is definitely not micro bubbles or sucking air in the sump.

1. Possibly pump cavitation, seems unlikely and would probably make smaller micro bubbles?
2. The elbow before my lock line has a small hole in the inside corner of the bend. It is meant to be a siphon break. I think at high speed it might be acting as a venturi and sucking air in gulps. Maybe I can find a way to temporarily plug it and see if that stops it. If so maybe making it larger would fix it, or I could put some kind of nipple that would stick just barely below the water line. Then it couldn't suck air, but should break the siphon pretty quickly.
3. The lock line is maybe doing the same thing, sucking air. I'd expect to see water able to leak out the joints if this were the case. I also used a turkey baster to wet the exposed parts and it didn't seem to have any effect.

Any other ideas?

 

[Diznaster]

It was #2 the elbow hole. I tied a strip of washcloth around it to basically block the hole, that improved it. I had a spare fitting so I tried drilling it out bigger, that made it worse. I made it bigger yet and pushed a piece of airline into it which runs just below the waterline, that stopped it completely. I'll do something a little less MacGyver long term, maybe even simply change the hole location to just below the water line.