If you have a fun activity like Free Choice at the end of the day, write that on the board in large letters. Whenever it gets so loud that you have to stop teaching to redirect the class, flip the lights on and off, remind the students to use quiet voices (preteach what this involves, talking about the difference between a close voice and a far-away voice, and reminding students that we can only speak quietly to those close to us during Centers), and erase a letter. Every letter is 30 seconds to a minute off Free Choice, and they spend that time quietly looking at a timer on the rug, then talking briefly about how to make things go better next time.
You can also look into a Yakker Tracker, but I have too many kids who would enjoy setting that off...