We've replaced the flapper, and flushing valve approx. a month ago. Ever since when it is flushed there is a loud knocking noise in the water line. Does anyone know what we can do to fix the problem? It's pretty loud. Thanks, in advance!

First check the linkage on the flapper. You should have 1/4" of play in it when the flappers seated. I'm sure you cut off the ring on the flapper and are just useing the hooks. That's what I would check on the drainage. The sater supply's a wee bit more involved. You may have developed water hammer. "Water hammer" happens when you abruptly shut off the water, as in your ballcock abruptly shutting off. You have at least 40 pounds of pressure pushing the water flow. The weight of the water hitting a 90 degree bend causes the sound that you hear. We put in shock absorbers,(air chambers) to cushion the hammer much like the shocks on your car. Back in the 60's we were required to to put shock absorbers,(air chambers) on all bath lavatorys and kitchen sinks. Over time water and condensate built up in them and they must be recharged. First turn off the house at the main house shut off valve. Then open hot and cold faucets at the farthest bathroom to prevent air lock.(This is important.) You will find under each fixture little brass or chrome valves. These are called angle stops. With a small pail held under the angle stop to catch the run off look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut. Put your wrench on it and back it off counterclockwise. Now open the stop and remove the stem and washer assembly, The stop will begain to drain. When it has quit draining reassemble the stop and move on to the next one. Do this on each one of your angle stops. There will be six of them. When you are done, close off the faucets you have opened, turn on the water to the house, and give yourself a pat on the back.
You have just recharged your air chambers and saved yourself a hefty service call bill. Hope this helps you out and HAPPY NEW YEAR! TOM

What style flush valve do u have? Is it a fluidmaster? Or ballcock?