I can't take credit for this one myself... I just found this idea on the following forum and it works fantastic!
Specific instructions and photos can be found on the above link, but here's the deal in a nutshell:
1. You want to make something removable that can seal off the top of the intake chamber... the little chamber above where the water comes into the skimmer.
2. The original poster used a lid from a margarine container. I used the top of a Cool-Whip tub. Whatever flimsly plastic you can cut will work fine. Cut out a rectangle from the plastic just about the same size as the chamber.
3. Take some airline tubing and cut it to lengths that will fit around the perimteter of the rectangle from #2.
4. Slit the airline tubing and slip it over the edges of the plastic rectangle. Hot glue in place if you want. I did.
5. Stuff the plastic rectangle into that intake chamber. The airline tubing will allow for a tight fit/seal around the chamber. You'll have to bow the plastic up a little to clear the intake tube. Just make sure you don't use too stiff of plastic and you'll be OK.
6. Sit back in awe of how quiet your skimmer just got.
Big thanks go out to "Drez" for this simple little 10 minute idea. I had no idea that all that noise was coming from that little opening!