I have this same problem in my 2014 GMC Sierra.
I had the dealer take a look at it, after giving them the heads up on the bulletin. They performed the steps that were listed but it did not fix them problem. After looking at it a second day, they came back and said it is the 'amplifier' that is faulty, and it would cost $740 to replace. Since I have 40,000+ miles, it is no longer under warranty.
Like others have mentioned, the chime is associated with the seat belts, so buckling up before starting resolves that part of the issue. However, the loud chime also occurs for the proximity sensors. It is also an occasional loudness as well.
The problem isn't worth the $740 cost, so I suppose I will just deal with it for now.