Power off in 5th brings a whine type noise. As does changing to 5th from 4th or 6th. Only 5th. Noise goes when I put power on. Why is this happening?

How old, how many miles, have you had it from new or did you buy it used?

Gear whine is usually a sign of wear or slight misalignment gears - power on and power off reverse the end thrusts so change the meshing point if the gears can move axially at all.