This is both frustrating and interesting. Xerox support seems unhelpful just saying stop it from going into standby! I have a recently bought June 2020 Xerox-6515DNI which I am otherwise very pleased with. I am sure the noise is coming from the switched mode power supply that is dropping into the audio spectrum when it is in a low power (and otherwise quiet) state. I think this is a well known phenomenon with these power supplies under low load.
For interest. Can I ask all those who comment on discomfort with this issue in future, to state their AGE (not DoB) as high pitched loss is usual with age. I am 62 and have good hearing BUT it was my 7 year old grandson who complained about the noise. I can hear it if I try. It seems to come mostly from the perforated window front bottom left of the printer. I wonder if some acoustic wadding over there would help? There may be one component e.g. an inductor coil that could be selectively damplened e.g. with a blob of glue, that might make all the difference.
Has anyone tried that? A photo would really help. I have not disassembled mine yet.