Since we've lived in our house for a year we hear thumping sounds in the PVC pipes in the concrete slab foundation. If I turn the hot water heater off for a day there is almost no thumping sound. If I raise the water heater temperature for a day the sound is louder and faster that night.

I tried to flush as much sediment out of the water heater. It initially would not drain when I opened the water heater valve. Strange thing is that it only thumps at night.

System has no expansion tank or water hammer arrestors.