Ray, 2 x 1" drains are probably not enough for a mag 18. I have a similar sized return, but have 2 - 1.5" drains in the overflow (1 will just barely keep up, in case the other gets clogged). When you add 90s into it, it gets worse. Since you are using drilled holes, I'd replumb in a larger pipe size or hose, everywhere but the bulkhead. The bulkhead will continue to be your limiting factor on the drain, but at least you won't be adding additional friction elsewhere. Those 90s are brutal.
To silence the sound, you need to get a piece of rigid tubing down to where the "burp" is being held up.
Bill
215g FOWLR... and anemones, GSP, gorgonians... carp, that isn't FO!
"I killed my first SW Fish in 1971..."