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Hi my name is ***** ***** I have been in the home improvement and repair field for 20 years. I will be glad to help you today. Likely the issue is that the condensation drain is restricted. There should be a clear tube that is connected to the base of the vent on top of the water heater and coiled up next to it. Remove that tube and clear it and that will fix the issue if the condensation backing up has not damaged the fan. If it has burned the fan out then the fan will need to be replaced but that is less likely.
Oh no. I am so sorry. I see Rinnai and automatically think about the water heater. I will opt out and open to other experts.
Noise is most likely the exhaust blower.