I have a 75 gallon tank with an overflow.I just filled it up and started the pump and now there is a horrible air noise coming from the pin hole at the top of the down pipe in the overflow.Is this normal for a while?
Try adjusting where the pipe goes into the water in the sump. If its too high, you'll here splashing in the sump, and if it is too deep, it will suck air from the standpipe. I had the same problem and that solved mine.
the home made durso works. I use one and have no noise. You can get the idea from the website and DIY for 3-4 dollars of parts from the blue or orange box stores.