I've just started to notice a metallic whirling sound coming from the front wheels when I take my Xterra out of 4WD. It usually goes away (or at least becomes much less noisy) when I drive the vehicle in reverse for a short distance. It also occasionally goes away on its own but there first is a loud "thunk". I'm bringing it in for the 22500 mile checkup pretty soon, but anybody else have that and know what it is?

Yep.

It describes the situation on the decal behind your driver's side visor....

You have to reverse a few feet to finish unlocking the front automatic hubs...otherwise, you are still engaged...

Leaving it engaged is used when driving over varied conditions, such that you might need 4wd for a stretch, and then go back and forth between 2ws/4wd on the fly.

So - Just back up after changing out of 4wd, and it will be fine, as described.

No worries!

I must disagree. There should be no unusual noise if all you have done is firmly move the transfer case lever from 4WD to 2WD. If you immediately reverse direction for a short distance after going from 4WD to 2WD, you may if you are listening closely hear a slight clicking noise as the front hubs unlock.

In my experience, "whirring" noises or clunks from the front wheels in relation to 4WD/2WD shifts may indicate a failing automatic locking hub. The failing hub will be on the side opposite where the noise seems to come from.

If you are going in for service soon, or even if you're not, ask the dealer to check this out while you're still under warranty.