Been running Beans for years and as mentioned: the drains should terminate just below the waterline in the sump, but some people run their open channel slightly lower than the full siphon or vice/versa I personally have never had any issues running both at the same level into the sump but there are many variables that differentiate one tank from another so I'd recommend playing with the length into the waterline, but not much. Your goal with the open channel is to have just a trickle of water spiraling around the walls of the standpipe. Anything beyond is too much flow(for the open channel)
From what you describe it should be resolved by playing with the standpipe termination level into the sump.
For the record I have my standpipes ending an inch into the water in the sump and it's absolutely silent.
From what you describe it should be resolved by playing with the standpipe termination level into the sump.
For the record I have my standpipes ending an inch into the water in the sump and it's absolutely silent.