Following on with
@Mooseman 's Logic... if the Rear End has a limited slip differential...GM requires a special additive to be induced with the Mobil1 75-90 Gear Oil...assuming it has not already been filled. It would be very helpful if you could take a digital image of your vehicle's RTO Plate (Driver's Door Jamb and or inside the Passenger Glove Box Lid... not sure about one being glommed inside the Center Console Lid) and post it here as there are people with the wizardry to quickly decode the features on manufacturing of the vehicle that are of import to our understanding.
I realize that your definition of "New" means just that... "BRAND NEW"... But in the case of these complex rear end gear boxes... you would be surprised how many of them are sold...and then wind up being installed...FUBARed and then returned and re-stocked on the shelves because the original purchaser failed neither to install it correctly... nor to set the proper "Gear Lash" between the rear axle Hypoid Gear...and the Differential Ring Gear...and damaged their working relationship as a result. In many cases... they flush out the gear box to conceal the metal particle damage and just put the case cover back...leaving the gear box...Bone Dry for the next unsuspecting customer to discover! One of our other Members... I believe he goes by @Dogfish24 ... is also known as "Chris Fix" and is renown for his excellent Youtube Repair Videos... one of which covers a How-To R&R the Rear Differential Cover Plate...Clean, Re-Seal and Re-Install the item and the how to properly fill the gear box.... This is worth viewing:
Unless this has been cleverly concealed... you should be able to determine whether anyone other than the factory assembler has been past the Rear End Cover Plate if you can see that all the original 10 or more Cover Plate Bolts have been removed and left evidence that this has occurred by the absence of paint around the Bolt Heads or shiny metal or damage to the bolt heads. Unless you discover some other reason for the clunking and bumping action first...keep this issue in mind... should you decide to investigate further the innards later on.
The following link leads to an RTO Translation Code Listing PDF for GM Passenger vehicles...and as sure as there are "Carts to Horses"... One or more of our astute members will chime in and provide you with the relevant one for trucks and SUVs. Still... this one is posted here to show how complex these things can get in de-cyphering the myriad Alpha-Numerics and it might still come in handy for those of us with other GM Camaro and Firebird F-Body Carline Series Vehicles for off-topic repair needs:
http://www.marshu.com/images-websit...codes/gm-firebird-camaro-GM-RPO-car-codes.pdf