A few years ago I installed a bathroom in my basement. When the fill valve in the toilet tanks shuts off I get serious water hammer. When I was running the piping I forgot to install an arrester in the stud cavity so I installed one later after the bathroom was finished and I noticed that I had a water hammer issue. I installed the arrester just before the shut-off valve for the toilet. After two different arresters and two different fill valves I still have a very loud hammer after flushing the toilet. I have no problems with the toilet upstairs and very little noise from any of the other appliances. The water pressure measured at the outside spigot is approx 66 psi. The bathroom is fed by a 3/4" line splitting off to 1/2" to feed the shower, sink and toilet.