4 wheel drive question

Hi,
My 2003 Silverado half ton has started a problem with the 4 wheel drive and I was wondering if anyone has had this experience.
When we put the 4 wheel drive in there is a fairly loud noise (as in the normal sound you hear when it's in, like a dull hum, but this is louder and sounds "dry"). Then when we put it out of 4 wheel it takes going down the road for a half mile to go out and then it will go out with a clunk. Any ideas?

Thanks,
Catherine

check your tire pressure and make sure tire wear is even front to back.if the rolling radius is different front to back the 4wd will bind up and destroy the transfercase. if the tires aren't the problem let the dealer fix it under waranty. i'm not sure about the 1/2 tons but the 3/4 tons have been having problems with the transfercase shift encoder motor.