Silverado 2500HD growling noise when coasting in 4wd

2001 Silverado 2500HD 44k
Today when I dropped it into 4wd mode, it's making a strange growling/scraping noise coming somewhere from the area around the 2wd/4wd
shifter in the floor. It only happens when it's coasting. When accelerating it gets quieter. I'm familiar with the hum coming out of a truck when it's in 4wd, and that's not it. Is it possible something might have dried up or gotten rusty underneath? It wasn't doing that a few months ago.
Any ideas?
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Rog wrote:

A reasonably educated guess would be that you are having problems inside the front diff. Because you have a 2500HD, you would have the larger front diff....we've had lots of problems with pinion bearings on these ones. The smaller front diff that you see on the 1/2 ton trucks and utilities is more prone to side bearing failure. We overhaul literally "hundreds" of these diffs in our dealership.
But you should have someone who knows what they are looking for to road test it and see if they can pick out the noisy area. I have also seen the bearing in the front output shaft on the t/cases come apart. Not as common as the diff problems, tho.
Ian
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Thanks Ian
That's sets me in the right direction.

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