The 01-04 LB7 duramax had a lot of injector problems with a 7yr/200k warranty on the injectors for certain conditions. Dealer also plays a part in the injector warranty on whether or not how willing they are to do it. some symptoms are knocking/rough idle, gray/white smoke at idle, hard starting, diesel in oil, etc.
We had the hard (almost no start!) problem with my buddy's Dmax while down in IP last year. First Rexburd dealer said it was because we didn't have the "Updated block heater cord" - In March. LOL
Second dealer replaced the thin metal connectors on the glow plugs. It got us home, but the truck wouldn't start the next day. All 8 injectors and pump replaced to the tune of 7600 dollars. GM covered the injectors, not the pump. Final bill was around 2k.
Get it looked at before it strands you.