Shawn Beltran Therrien

I was thinking of getting a 6.0 gmc that had 72k miles on it. I took it for a test Drive and once I reached about 65-70 mph I would hear rattling in the engine bay and the check engine light would be flashing. The guy told me that it was back firing and I said that’s Bs I don’t think it is. Would anyone know what exactly would be making that sound cause I have a few theories just not sure.