Loud brass valve - how to make it quieter?
A &quot;globe valve&quot; would be more suitable for the task. Both ball valves and gate valves are intended primarily for shutoff - either fully open, or fully closed.
A likely noise source is cavitation (bubbles forming and then collapsing) at the sharp edges in the flow when partially open. A gate valve has a plate with a (relatively) sharp edge that will be in the flow if partially opened, as does a ball valve.

A globe valve is more oriented to regulating flow at points between on and off. The opening is a ring that gets larger as the valve opens, without having part of the pipe wide-open for direct flow while another part is occluded by a sharp metal edge. There IS a sharp edge there, but it's presented equally to all the flow, and the valve design slows the water going through the partially opened valve.

You could also try running all the water to the higher waterfall, and putting the valve to feed the lower waterfall there, since you have excess pumping capacity. The additional pipe distance (friction head) to the lower fall and having already pumped up to the higher location would reduce the imbalance between the flows if they were split at this point, rather than at the lower falls. With some experimentation, you might also try a smaller diameter pipe to feed the lower falls, creating additional friction head (attempting to balance the additional 24&quot; in real head to the upper falls, or at least put less of that job on the valve.)
Valve images are from wikipedia