+1 on the oil pump possible cause, any pinging or knocking noise from the engine is not good, could be the lifters bleeding down due to low oil pressure, I would seriusly stop driving it to prevent further damage, and if did get damaged, make it an excuse to do a stroker kit....
Had a 99 GMC ESCB with LQ4 conversion and Nelson Performance tunning, currently workin on my 82 SCB GMC with carbed LQ4 conversion in progress, Also have a 00 SS Camaro that runs mid 11's all day long with bolt ons, Drag race my 79 malibu that has a 496 BBC runnin 10's on motor, shootin for 9's with the 250 shot of nitrous in its near future. Use my 04 2500 chevy to pull my rig around and drive the fam in our 07 NNBS yukon, Die Hard Chevy Fan Till The Death.