Engine has a knocking sound.

Yes the sound changes. Pulled plugs. A lot of miles on them/ changed them.

Not nessesarily, sound changes because cylinder isn't firing. I am a journeyman autobody technician. I'm car wise and I can tell u its coming from top end. The oil is dirty. The engine shut off because it ran low on oil, but new rod bearing were installed. Trans ran low on fluid, engine reved a bit high 6500-7000. Thats when the knocking. I'm up close and the top end is making the sound. I added some new oil and it did quiet it down a bit but just for a few moments. Its not a deep low end noise. Almost, but little higher pitch top end. Lifters I think. I really appreciate an experienced auto mechanic not technician. School to start but when it comes to cars, mechanic means they can hear a noise and they'd be 98% correct with the answer. You can't beat the experienced old timer. Worked with a few(now I'm one)? I'm not putting u in that category, realized your header says tech, nothing personal if you're human. Any input is appreciated. Thanks

Ok, 1999 Saturn sl2, dual ohc, my mother bought it brand new. It has 57,000 original miles. I put the 7,000, it ran fine until that shop messed it up. I told him off(the manager) anything else.

I think u are spot on! Now the dyelamia

Hi, I just wanted to express that I'm car-wise. Didn't mean anything towards him. Hope u understand. I learned antibody by experience not school. Its making different noises. I plug the hose between the air filter housing and valve cover and it quiets down, except when I give it gas then top end knocking and a noise lower end slap.