I would do a "poor man's herbie". Maybe run a 5/8" or 3/4" vinyl tube from upstairs to the basement sump, tie wrap it to the side of your existing pvc drain on the way down for support. Put a valve upstairs so you can control the flow with this new tube under your stand. Only way to do it, over the rim with a small "U" into the overflows built in. You want "most" of your overflow water volume to flow down this tube, with no air in it. Then, continue to use your existing pvc pipe overflows to take a tine"trickle" of the left over water. This will make it dead silent when you adjust it to the right amount of flow in your new vinyl tube. You will need to "Y" the vinyl tubes to each overflow. Then the worst part is to start the flow, go downstairs to the sump to start the siphon with a big "suck".
It's loud because you have air in it. You need another route for a large majority of the flow, it needs to be solid water the whole way down. No air. Only way to do it without cutting into your existing drains and adding another "sub sump" under your tank, which I personally wouldn't do. Since your existing drains are so big, no problem using a smaller tube to rely on. If it malfunctions, your existing drains will work how they do now, you'll hear it, go downstairs and restart the siphon again (and cut the flow a tiny bit then, losing siphon means it took in air".
Hope this made sense.