I recently replaced my water heater with a tankless unit (Rheem). After installation I noticed that whenever I turned on the hot water; or when my central heating unit would turn on, the tankless unit would make 3-4 loud knocking/popping sounds. I asked the plumber who installed it and he said "no idea" [gee thanks pal].
Today I replaced the gas pressure regulator thinking that might be it. The old one was rated for 100k BTU/hr while the heater needs 180k BUT/hr [you'd think the plumber would have done this]. The new regulator is rated for 200k BTU/hr, but the water heater is still making those sounds!
I don't want to open up the unit because I really would have no idea what I could do. Does it have to do with the water line? If so, why does my central heating unit make the water heater start knocking? Does this sound familiar to anyone?
EDIT: Here is a recording of the water heater after the faucet has been turned on.