If I had to guess though I would guess it was a sticky lifter or lifters. If it wasn't serviced for a looooong time I would say it was because of dirty oil that got your lifters ticking.
The oil is dark dark black, it's just not good. So I'm gonna do an oil change this weekend and run some seafoam through the engine, so if it is old dirty oil making my lifters tick hopefully this will clean it out.