99 Ford Expedition Transfer Case???

I really don't know anything about cars. My husband is currently deployed to Kuwait. In March 2008 our transfer case was replaced by this shop but they bought and installed a used one from a junk yard. Well ever since I picked it up it's been making like vibration noise underneath the car....and I took it back but it never made the noise. Well, today I had another mechanic take a look at it and he told me he thinks it the transfer's case b/c he can hear something in there thats loose. I went back to the shop and they told me they didn't think it was but if it was the warranty was only good for 30 days. So what I want to know is it normal for something to be loose in the transfer case? The Vibration noise isn't like just a quiet vibration its loud kinda sounds like the car wants to break down! If anyone knows anything about this I'd really like to hear what you know. I don't want this shop to scam me just because I'm a girl that doesn't know about cars.