Are we sure it's not piston slap, like in all the GM Vortec engines that have had their piston skirts removed?
My '06 Altima developed this at 142k, sounded like a VW TDI when cold. After it warmed up everything went quiet, didn't use any oil either
If the main bearings are going bad make them show you. Simple drop of the pan, remove one main cap and inspect.
Flipper is onto something as well
