I got this idea from this list so thank you to whoever thought of it:
I write the word "NOISE" on the board. As the noise level gets louder I
start erasing letters starting with the letter E.....they get three
chances...when the "E S and I" are gone there are only the letters "NO"
left----and I tell them there is NO more talking during class. Usually just
writing the NOISE sign gets them thinking about the noise level and they
start "shushing" each other when it gets too loud.