mbox and rode nt3s making high pitched noise

[Kurt Walker]

hey all

i hope this is the right place for this....
i have a fairly new in terms of use mbox. i have several mics which i have used with it a bit but only on condenser...the rode nt3. when i turn on the phantom power at the mbox or on the mic from the internal 9 volt i get a high pitched through playback. i have two rode nt3's and both exhibit this behavior....do i have a faulty mbox? i mean i have rarely used it and never dropped it.....any ideas would be great and i am new to this so sorry if it sounds like an ignorant question.