I wouldn't necessarily recommend any modifications to your existing overflow plumbing without knowing more about it. Is this a prefabricated standard overflow sold with the aquarium or something that was added after the fact? Describe your plumbing... more details needed (or even better if you have pictures). There are many different overflow setups and noises can be generated by a wide variety of things (i.e. air running through the plumbing, in the overflow box, too little flow from the return pump, etc).