Could be a piece of carbon caught between the head and piston. If it overheated, that could be it. Water or Sea Foam through the intake at high idle will knock the carbon out. If its not carbon, sure sounds like a rod knock.
I respect you being a new member, but post this belongs in the "THE NEW TURBO REGAL" forum as the people there have experience with the new generation Buicks as most of the "old timers", like me here, have no clue on any Buick after 1987!
To be fully truthful, I did go so far help a friend change the brake pads on his 2006 Buick.