My truck is making a knocking when started and then stops after driving? on 2000 GMC Jimmy

We bought a 2000 GMC Jimmy a couple months ago. Will never buy another after this one! Been nothing, but problems! Anyways, last I visited my father, getting ready to leave I started the car and what sounded like knocking. The oil had been a quart low, after putting it in, my father (a mechanic for 40 years), said maybe it was from that, but went to start it this morning it did it again. Oil level was fine. After warming up, it stop doing it. What could cause it to do that?

Asked by for the 2000 GMC Jimmy
poss broken flex plate.the noise goes away after its in gear? poss lifter issues?
Once in moving, sorry didn't notice if it stopped after in gear, but even sitting warming up. Once warmed up it stopped (without being moved). Talked to my dad this am and he (and my husband) said they think it's my lifters. Will let you know if we figure it out.
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