I recently plumbed in a new Noritz tankless heater. Besides having the typical "takes a while to get hot water issues", mine also makes a "long horn" sound upon start up and than again when the faucet shuts off. I haven't had the chance to contact tech support as I haven't been able to get somebody to turn on the water in the house while I sit with the phone outside with tech support. The horn sound is a bit annoying, but what's even odder is when I flush the toilet, I hear the horn sound after the flush is over. The toilet is connected to cold but is there any way that a low flow toilet pulls enough cold water from the house pipes that the cold feeding the tankless sets it off? The tankless should only be sensing flow needed from the hot side right?
Thanks for helping with this quirk.