make a bigger overflow....
but seriously, you could try doing a modified durso (ie Stockman style) They fit in a more compact space and you will get the same result. other than that theres not too much you can do. you may try putting LR rubble in the overflow chamber. that would work if the noise is from the water falling and splashing in the overflow itself. But if its slurping from the standpipe, the durso is about the only fix that is universally effective.