The pipes inside the overflow box is not the part making the noise. Its the area where the water first enters the sump. These pics might help show you what I am talking about.
This pic is of the right hand side overflow box. The water level seems to be too high for some reason.
This is the left hand side overflow box. The water level in this one is a lot lower and seems to be perfect.
Here is the part that is making the noise. And its not the waterfall sound thats bothering me. its making a very loud bubbling sound like boiling water. As you can see the water coming out the the tube on the right is a lot stronger than the one on the left. On the left tube you can still see the grate where the water comes out. The sound is coming from the tube on the right and that tube is connected to the overflow with the high water level.
If there is no way to fix this then what is the best and safest sound insulator I could use to block up the back of the stand?