From the link:
"When the engine is idling if you rev the engine gently and it makes a rhythmic knock on the overrun or as its accelerating then 9 times out of 10 it’s been run low on oil, the bearings have made contact with the crank (metal on metal) and this has worn part of the bearing metal away making the clearance too large for oil to fill successfully. So that knocking noise is metal hitting metal."