2000 denali piston slap

2 weeks ago my 2000 denali gave me a bit of a problem, when i tried to started in the mornings and after work, it felt like it was forcing itself and there was a knocking noise right after, it would later go away after warm up, after 2 doses of chevron techron the starting problem its not as bad but the knocking noise still there, i spoke to the mechanic and it seems like theres carbon residues in the oil, i changed the oil and filter right after, still knocking, it sounds like a slap when i accelerate. how bad could this be? would an engine/oil flush do the job?am i in the right path....battery ok and clean, no check engine,fluids ok. any help will be greatly appreciated