2001 GMC Yukon XL 5.3L: Deep grinding noise when put into drive or reverse, when driving or stationary... help?

[Yukont]

My truck is a 2001 GMC Yukon XL with the 5.3L Vortec and I also know that it has a Eaton Locker 2WD. 217,000 miles with a rebuilt transmission.

 

Two other things to note is that it's a slight noise and not a super loud noise... and the noise seems to go away over 10mph.

 

So basically whenever I put it into drive or reverse, I hear a deep grinding sound... mostly when stationary. I changed the rear diff fluid with Mobil 1 75w 90, 400 miles ago. I'm not sure if maybe changing the diff fluid caused this or not but it just started making the noise yesterday. I need help on what this might be? Any insight on what this sounds like to you would ve greatly appreciated. Thank you

[silveradosid]

check flex plate and torque converter bolts