I think you have a cavitation issue, not in the pump but one of the PVC fixtures. Guess somewhere between the elbow and the inlet of the pump. Lower pressure or increased flow rate will cause a cavitation.

Any change if you close the inlet valve a little bit? Not too much.

I wonder...Something you could try, since your bulkhead is low enough to allow you to have a wide water level range. With just that loop running and both valves fully open see if the noise differs between a water level just above the bulkhead and the water level kept at the top of the tank. I'm not sure that raising the water level would increase the pressure or not. It's just something I would try if I were having the problem. Doesn't really provide any solution, but might show you're cavitating if the noise lowers if the water level is raised.

And then hmmm, I think someone suggested some eductors on the discharge of the manifold. Would that produce a similar back pressure like closing the valve?