2000 Ford Expedition

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It probly is your transfer case fluid. My four wheel drive was making that sound in 4high. Look at the transfer case there should be a drain, fill plug. May look like a bolt loosen it if its at the right level you should expect to see some drain out if you can see any in there then even so you may not want to add more fluid I dont know how many mile it has been since last changed so it might be low and need a change its not to exspensive to have it changed if your a do it yourself person then be sure to drain all old out and replace with new I hope it stops the noise for you.

Best of luck.