I have a hang on type overflow box. i ran a length of airline tubing down it far enough until the noise stopped, then clipped it to the side with a clothes pin.
I have also seen people use a gate valve on the overflow also, of course alot of people will disagree with this, but as long as you close it just enough to cause a *slight* backup in the water flow, it should be fine.
I know people are against bio-balls lately, but I have no noise problem because I have bio-balls in my overflow with a filter pad stuffed over them.
I also have had 0 nitrates since I started this method. I don't knwo if there is a link, but at least it proves that the balls don't CAUSE high nitrates.