Some of us can hear certain frequencies that other people can't. My brother once had a Television that would drive me crazy. He could never hear the sound. Some people will say that most Televisions make that same noise. (This was more common back in the tube type TV days)
I've had one of those water heaters. The circuit board makes a noise. All I found that worked was to put the circuit board behind a thick wood panel that would then absorb the noise. Maybe his is louder then mine was. Maybe his hearing is more sensitive then mine.
CT51 is not crazy or imagining this. Some circuit boards make a noise.
CT51. You can purchase a new board relatively cheaply. The new one might be quieter.