Bradford white tankless water heater

Hello, I have a gas tankless water heater on a job, it works fine but when it is “off” or on standby in between uses it will intermittently make a noise that won’t stop till the heater is either reset at the unit or another faucet Is turned on and it cycles again. This usually happens after some one has a shower and the unit is ran for some time. It is never used at maximum capacity or anywhere near and it doesn’t have any trouble codes. The house has 2 of these exact same units and the other one does not make the noise. I am stumped and so is Bradford white as they have helped trouble shoot this problem and can’t find the solution. Has any one ran into this problem or a similar problem? It is not the exhaust fan staying on or anything like that it is a noise that could be described as a beeping noise or a humming noise that’s changes frequency’s. Any help would be appreciated.

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