Hmm I have the same problem but I can actually get into my overflow, the sound of the water rushing down the triangle overflow and hitting the bottom is quite loud and the sound of the water hitting the filter media below as well.
I was planning to make a spray bar type arrangement at the bottom to stop that noise but was unsure how to stop the noise in the overflow, whats the ball valve supposed to do?
Also my plumbing has a tap in it which I can use to adjust the water flow from the pump, is this bad for the pump because I close it a little to stop the water building up too much in the overflow and gurgling through the pipe into the bottom.
I dont have the sump running at the moment because its in the lounge and too noise just have an external filter on the tank but would love to get the sump going just keep forgetting to post so im sorry to steal your post Jude but you reminded me.. :oops: