2001 Silverado: Howl/whine in 4wd

I have a 2001 Silverado 1500 Z71 with the 5.3 liter engine and the automatic transfer case. It has almost 36k miles.
When I put the transfer case into any 4wd setting (auto, hi, lo) I get a low
howling noise from the front of the truck. There is no noise in 2wd. The noise is loudest while accelerating but is also noticable when coasting to a stop. The noise is also louder in 'hi' than in 'auto'. I just bought the truck so I don't know if it has always done this or not.
A Chevy dealer (Cambridge (MA) Chevrolet) replaced a hub which didn't change anything. They wanted to rebuild the differential for $1200 - $1700. Instead, I took it to an independent mechanic and they think the noise is normal transfer case noise. I also had a GMC technician drive the vehicle and he thinks it is normal.
Any advice for me?
Thanks!
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Tim Hall wrote:

Yeah,
All three of my last GM's did this...
My '00 Blazer had a mild howl, all of it's 98K mile life.
My old Avalanche did up until about 40K on the clock, then it almost completely cleared up.
My current truck, an SS, is making the noise too... but it's mainly while turning, because of the AWD setup.
In both of my old trucks, I just ignored the noise and never had any issues. All fluids were relatively clean w/no metal shavings or anything ugly like that in them.
I'd have to say that it's normal...
-marc
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"Tim Hall" wrote

change
Sounds like you have already had a number of opinions, are you just looking for more?
Personally, I'd go with the original Chevy dealer. It's not normal for these trucks to howl when you put it in 4hi, but I see and overhaul bad front diffs in these trucks all the time. It's almost an epidemic in our shop. Usually, the side bearings go out, you get excessive backlash, which takes out the gear set.
Has anyone pulled the drain plug on the front diff? Often, when they are coming apart, you will find all sorts of metal stuck to the magnet on the drain plug.
Ian
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shiden_kai wrote:

Nothing unusual came out of mine... Just darkish fluid.
-marc
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