My wife got me a new 2008 Silverado LT for Christmas, back in December of '08. It's a reg. cab, short bed, 4.8L, Silver Metallic, runs like a dream, and I love it. Today, 41,000 miles rolled up on the odometer, and all's well, except for an issue that's beginning to get on my nerves a bit. For about the past 5,000 miles, I'm getting a distinct growl or "whirring" sound from the differential. Loudest when warmed up, and between 45 and 55 mph. Pulled the plug (magnetic) on the diff. cover, it was loaded with metallic "mud". Decided to go ahead and change the lube, pulled the cover off, drained the original lube and inspected the interior. Everything's clean, no metal shavings, and the old lube was also clean and clear (purple color). Put Mobil 1 synthetic gear lube back in it. (It's the G-80 locking differential). I hate to do it, but I suppose I need to take it back to the dealer and see what they will do about it. I don't think it's a rear axle bearing. It sounds like maybe a carrier bearing that's gotten rough. (Had this same issue with a Ford F-150 I owned, over 30 years ago). I've had many other GM trucks thru the years that have had that "rear end whine", or gear whine that is so common with high-mileage vehicles, but this is definitely a growling sound, almost a low "roar". Question; anyone else out there having this type of issue with their Silverado 1500 pickup?