sump/refuge questions help

I have a brand new setup here, and my first salt water project. my main tank is 90 gal with a Megaflow over flow, and is curiulated by 2 Kor 3s,,the sump is a 40 gal. L. it has the skimmer, return and refug in it.the water exits the tank and down into the skimmer section. on the way down I have a T to send the water over to the refug on the other end , then they both dump into the return and up into the main tank. Both the skimmer and return are pumped with Mag 5s Heres the problem, when I turn on the return pump the water goes up and out into the main tank and the comes down the siphon, however it is making a loud air bubble sound every three minutes or so. Kinda like a f**t in the bath tub. Also it is making alot of bubbles. The skimmer is not on , and by the way it is a EV 120. The Magaflow siphon, is the water supossed to cover the two small holes on the sides? Also there is no water at all going across to the refug. Please look at the pics and give some suggestions. I will try my best to answer all questions. PS I added a pic of the main tank with the water in it.

 

So mor pics

 

I noticed a couple things from your pics. First off a mag 5's flow is kinda low for that tank I use a quite one 3800 or 4000 that gives 780 gph or around 600 gph at 5 feet. Same with your K3's I would consider getting another one if you have algae problems. As for the noise you can consider getting a durso stand pipe. Iread some where though that the bathtub noise is cause by having the outlet for you overflow to deep in the water. As for why there is no water moving through your refugium. The water is falling straight down you need a ball valve on the part that goes straight down to divert water to your refgium. Great set up!