In our master bath, whenever I flush the toilet, or any toilet for that matter, there is a rather loud (single) knock that you can hear, which sounds like it is coming from the shower valve/lever area. I was told originally that it could be a water hammer and ended up getting water hammer shock absorbers installed, but it did not work. Someone suggested that it could be the shower cartridge needs replacing. Any ideas/suggestions as to the root of this annoying problem would be greatly appreciated.

It is still likely water hammer. Can you tell us exactly where you installed the arrestors? Did this start right after installing a new flush valve or any other plumbing work?

The arrestors were installed on the hot and cold pipes leading up to the shower from the basement. I did replace the flush valve in the toilet, which is an American Standard 1.6gpf, but I did have the hammer prior, which was a ticking sound after the toilet flushed or the taps were turned off (still occurring in the shower pipes). It has now developed into the knock. Could the problem be the flush valve then?

Check your water pressure. Let me know if you don't know how and we'll walk thrught it. Your pressure should be around 45 psi.

Found that same problem today, customer said after she flushed the toilet there was a bang. Well I found the waterlines in the basement needed to be strapped and some new hangers installed and the bang went away. I had to move the ballcock float up and down in order to get the noise she was hearing. Good luck Check for loose pipes and some pipe insulation can be used to help keep them from knocking.