Did it have an induction kit, or the standard air box ?
i cant comment on a valver with an I/K but mine was standard, with the standard air box & it did not make any high pitched noises at all.
Bit of a long shot - did the car have an amp wired up?
as if you put the power lead & the earth next to each other (down the sill) that will make a high pitched noise with the revs, the high pitched noise is interferance between the two leads.