Im intimately familiar with the Gates product line and belts in general (working for them and selling them for 25+ years kinda does that)...
Ive recently spoken to our guys about this issue and the only solution is to change the tensioner and the belt. The noise comes from misalignment due to tensioner failure (or developing failure) and also by tolerance stack in the mounting components (brackets, tensioner, engine block casting, etc). Max acceptable misalignment is 1 degree (about .060") per foot of span length (the unsupported area between the idlers). The 6.4 drive layout is especially prone because of the very short runs between grooved idlers at/near the tensioner. For those who have done this change, you've seen how short these distances are and can now understand the issue.
On the belt, we have one OEM construction that is a lower noise compound but don’t currently offer it in our Green Stripe replacement line. Im not aware that our competitors have one either. All belts are built just slightly different and these differences can sometimes make a difference in how the belt sounds--for a little while. Remember, no matter what belt you use, and have misalignment, the noise will eventually come back. Manufacturing investments are big with some belt construction changes, so I'm not sure when/if a quieter belt may be available in the replacement line.
