I just set up my 240 and have it drilled with two bulkheads and pvc. I cant believe how loud it is!! My 90 gallon AGA was pre-drilled with built in overflow. It never made a sound! I am going crazy as well.
I used clear vinyl tubing instead of the tall pvc pipe b/c the tubing would bend to one side of the overflow box so that I could get all the fittings at the top to FIT in the box!
frankb, where exactly is the noise coming from?