So the water is making noise as it drains through the weir into the internal (inside your tank) box?
If that is the case, sounds like your bean is draining a little too fast. You should be able to slow the main drain down some. This should raise the water level inside both the boxes some.
If that is the case, sounds like your bean is draining a little too fast. You should be able to slow the main drain down some. This should raise the water level inside both the boxes some.