Strange Air Leak

I am having a strange problem with bubbles coming out of my return lines....

When my waterfall is off I get several tiny little bubbles that you can sort of see if you look closely out of the returns. When my waterfall valve is open I get a lot more bubbles out of the returns and you can see them noticeably on the surface. I looked inside the pump window (where the basket is) and there are NO air bubbles inside it. The water level in the pool is perfect, and I don't have any suction side cleaners that could be leaking. I assume the leak isn't in the line from the skimmer, since there would be air in the pump, so where the heck would it be coming from? Any ideas on how I can diagnose it?

For background info - I have 1 skimmer, 2 returns, and a waterfall. No main drain.

The water goes in the pump, then through filter, and then goes out a pipe that branches into two pipes w/ an adjustable valve - one feeds the returns, one feeds the waterfall. The waterfall line goes right out to the waterfall. The return line goes into my heater + salt cell and then back out to the pool returns.

Any help or ideas are welcomed. I am stumped.

Thanks.