Squeak from rear upon slight acceleration

I have a 2015i with 8 spd transmission. Tranny has been replaced to address the "clunk on take off" issue. I also had, and still have, a chirp or quick squeak coming from 3rd row area that happens consistently when throttle is applied slightly accelerating gradually from a coast. Most notable in town around 30 - 40 and easily duplicatable. I drove my brother-n-laws 2015 Tahoe and it exhibits a slightly lower chirp doing the same thing. Dealer has heard it but cannot figure it out. No TSB on it either. Anyone else have this issue and found a cure? It is not the same as the high pitched whine coming from diff control unit that has been reported in a few threads although my Escalade and my b-n-l's Tahoe both have this as well and there is actually a TSB on it with a "no fix" resolutions from GM. Thanks!