There are parts inside the transmission that still rotate when in neutral, so it's possible. However, I'd suspect an accessory is making the noise. Power steering pumps seem to have a way of making all kinds of odd noises for no particular reason, in my experience.
Remove accessory belts one at a time and start the engine. When the noise stops (if it stops), the last thing unbelted is the noisemaker. Do the alternator last so you don't overwork your battery.