I have a 2021 GMC Sierra 1500 Denali Duramax 3.0 diesel with 13000 miles. I have been experiencing a knocking noise when I reach 20 mph and it continues to 40 mph. The dealership says they don’t hear a problem. Does anybody else experience this? Is there a fuel additive or oil additive that i should be using that may help? I’m new to diesels so suggestions would be appreciated.