when i test drove another z in another color that i ended up not buying, it had the high pitched noise. i have good ears so i could hear it clearly, but it wasn't to a point it was bothersome. it was defintely there though.
it seems some cars are louder than others?
if it doesn't bother you i'd just leave it as is. it's not like the z is any luxury car anyways and it has plenty road noise.
my z does not have the high pitched noise.