So quick little story of how this came to happen . My buddy and I did a pull in his 98' wrx sti . We pull back into the parking lot and we notice there is a knocking noise . We drive back to his house a couple blocks away. The knocking or rattling noise only came up at around 2000-3000 rpm . We check fluids in the car and turns out this idiot has barely any oil in his car. We fill it up and run it again . knocking still persists . we decide to replace his filter as well as that was overdue and see if it was a clogged oil filter not getting oil to the engine. We test again and the knocking is still there but now its happening during idle as well. I check the if it was just rattling heat shielding or my downpipe , nope wasn't it. So now I'm wondering what else could I check to see what the cause could be . Plz halp 
