I too have a LifeReef single overflow, and found it a bit noisy. I'm using a Mag 7 for my return pump.
At first, I used Jeff's suggestion of hanging a piece of air tubing down in the standpipe. This works; you just have to fiddle with it a bit until it quiets the flow down. The tubing doesn't restrict flow at all; it just breaks up the noisy vortex. I found that the tubing doesn't stay in the optimum place, and unless you secure it, can slip down the standpipe.
I stopped using the air tubing, then took a rectangular piece of 1/4" plastic and simply laid it over the top of the outer box. It rests up against the U-tube, covering all but the last 1" of the open top of the box, which is where the U-tube is. It cut down on the noise a lot.
One of these days I'm going to trim it so that it covers the whole top while just allowing room for the U-tube itself. It definitely helps as-is though.
I am however intrigued by the standpipe mentioned in the previous post. I don't like the idea of losing the prefilter, so if I experiment with one it will be a modified something that works with the prefilter still there. If I get it working I'll let you know. Darn it, it's 3:22 AM, and now I'm curious to see if it WOULD work... ARGH!
One note: I did a lot of reading and research before buying an overflow. I have found the LifeReef overflows to be everything they were reputed to be, and more. They are topnotch in every way. The noise issue is a very minor one, and would be present with other overflows as well, except those which move the water too slowly and need powerheads to keep them going...