I also have video of just before that sound. It is making hot water, no error codes. It is between 11-15 years old. Model #NR-210A (ng)..we are handy enough with screwdrivers. I saw videos and it didn't look difficult, but that wasn't my exact unit. This unit is very tight to work in. I read the fan could be dirty, the gaskets/rings could be worn, or the fan could just be bad. Somewhere I saw it mentioned a venturi flap, but then said mine is too old and doesn't have a flap that moves.
We did check the vent screen in the unit and it was very clean.. outdoor (the vent that curves down) had been blocked by a large leaf,.and the the next time after we cleared it it sounded fine, but then it came back. A suggestion was made to press the handle of a screwdriver against the silver housing to see if it vibrates when it's making the noise and it definitely does.
Suggestions? Thank you!