The last couple weeks when i start my truck in the morning (without block heater plugged in). It sounds like my sisters 99' 2500. It really knocks for the first 2-3 minutes. It still run the same, no codes. Is it something i should be worried about? Any possible causes? I got this truck last winter and it never did it then. Other than having all 6 injectors replaced and i.p. at month 1. Thanks.