One last post on this issue. In the interest of documenting what we have found in our effort to identify and correct this problem, I offer my final thoughts.
1. The problem: Pulsating sound (pounding/hammering) in piping after flushing any of the three toilets in the house. Opening any other valve in the house has no effect (only toilets cause the noise). Noise stops when toilet fill is complete.
2. Findings: System has an installed PRV (Pressure Reducing Valve) which should maintain PSI between 50 and 75. System at 80 PSI (desired 50 PSI). Pressure could not be changed by adjusting the PRV (defective PRV).
3. Actions: Replaced PRV (see previous postings regarding original and replacement part Numbers).
4. Results and conclusions: Noise corrected by PRV replacement. Original PRV was 20 years old and was not reducing street PSI. System was over pressurized by 25 to 30 PSI. Initially, we were confused as to why only the toilets caused the noise. After PSI replacement water flow at showers and other faucets seems to have increase in-spite of the lower pressure. This fact leads me to the theory that the PRV was not only failing to reduce the pressure it was also restricting flow while water was running. Probably because there was materials caught in the valve restricting water flow. This would explain why only the toilets seemed to be affected; IF the toilet float valves were affected by the changing water pressure. (The pressure in the system was observed to fluctuate between 45 and 55 PSI while the noise was occurring). Suspect the float valves were responding the the pressure changes by opening and closing slightly compounding the hammering noise. All three float valves are of the same type, manufacture, and age. Hope this review and explanation will help and save others from the same malady.