I have a 90gal with corner overflow as well. This is what I did. Drill holes in the top of the drain pipe. Use a small drill and drill a hole...listen if not happy drill another small hole until the noise stops. Also make a cover for the top of the overflo. I used a piece of black acrylic.
If that doesn't work. I think the Syncra 4 is closer to 900+ gph. If you have one of the 90g tanks with the megaflow, I think the flow on them is closer to 600 gph. Maybe try dialing your return back a little bit.
Google a Herbie drain set up which works wonders if you have 2 bulkheads in the overflow. Typically one was used for a return pipe but you can just plumb that over the side of the tank which is well worth the extra work in order to get a much quieter tank...