'13 Sierra front end noise

[bpm13006]

How's it going everyone? So I recently purchased at 2013 Sierra ECSB WT spec w/ 60k on it and I absolutely love the thing. I was out of state when the truck came up for sale, so I had my brother look at it and eventually make the purchase for me so I wouldn't miss out on the deal I ended up getting. Long story short, I've had a few issues. My lights would randomly turn off while driving and my doors would occasionally lock while cruising at highway speeds, but I tracked that down to a bad ground on the aftermarket remote starter. My current issue is a strange noise coming from the front end. Describing it is difficult so bare with me: when accelerating from a stop, somewhere between 2k-3k rpm I hear a rattle coming from what sounds like right below the transfer case shifter (my WT model has a manual floor selector). The problem also makes itself known when the truck is absolutely pinned and at the top of the rpm range (I occasionally run into this merging onto highways). The best way to describe it is a handful of ball bearings in a washer machine. I've had a buddy who's a GM tech unofficially look at it and he was unable to diagnose the issue. All we know is that its drive line related because the noise doesn't occur at any rpm while the truck is in neutral.

 

Anyone have any thoughts as to what could be causing this?