you can always get rid of the noise by turning the ball valve, but it's at the expense of decreasing flow.
You need to work on the overflow side and try to get that as quiet as possible in order to keep your potential flow maximized.
When you eliminate the noise by closing the ball valve, you are slowing the flowrate down enough to allow the overflow to "pour" the water through the plumbing. The noise comes from forcing water through the plumbing from a siphon. This causes air to be sucked in as well and gives you the gurgling toilet sound.
Options to help reduce the noise:
- restricting air intake by finding the right sized hole
- standpipe height (this will help with the build-up and flush cycle effect)
- adjusting the depth that the plumbing is underwate where it enters the sump (aim for <2")
Also, eliminating right angles on your overflow plumbing may help as well (use two 45's instead)