Diff whine
If the gears have recently been installed and if yes, assuming they have maybe less than 50 miles, reset them and hope they run silent. The more miles on them assuming they are not set up correctly (resulting in whine), the more likely they are shot and will need to be replaced, again.
i'm guessing if they've been set up for a while and they haven't made any noise until recently you probably have a bad pinion bearing or carrier bearing, i wouldn't drive on it long until you get them replaced because you will put uneven wear on the ring and pinion and when you get the bearings replaced the gears will whine afterward, if i had my guess i would say most likely a pinion bearing
