Water hammer problem with toliet valve

I've had one of those fluid master toilet float switches for 4-5 years with no problems. But then it started to make a loud bang (water hammer) when it would shut off. I replaced it with the same type. After about a month, it did the same thing. I returned it for another and again, this one worked fine for about a month until the water hammer started again. I've since turned the toilet supply valve about halfway closed to limit the water flow and solve the problem. But any ideas as to why this is happening?
1. We have good county water and normally do not get any buildup of deposits.
2. It worked fine for years, and the other bath rooms are working fine too (single story home).
3. No water hammer problems with showers, sinks, etc. - only this toilet and only recently.
Thanks
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Based on my experience, it's possible that the oncoming water pressure has changed, and that only one plumbing device is sensitive to it. Or, if you have a pressure regulator, it may be having health issues, getting ready to fail. Got something like this living on your cold water pipe?
http://i27.photobucket.com/albums/c197/ancientangler/PressureRegulator_02.jpg
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To prevent water hammer, builders will put a copper stub right at the faucet (may be in the wall). The air in the stub acts as a shock absorber. Over time (5 years is long enough), the air will dissolve into the water and the stub will become water logged and unable to do its job. The simple solution is to totally drain your plumbing and reintroduce air into the stub. IF you don't have a stub, there are water hammer arrestor devices yiou can add to do the job. THese devices have a bladder which prevents the water logging problem (unless the bladder breaks).

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