Engine knock after over heating. Only knocking  when under load. What could it be?

Engine knock after over heating. Only knocking 
when under load. What could it be?

My friend bought a truck from another friend of 
mine. Long story short he sold it because he blew 
a lower radiator hose and overheated and now it 
knocks when under load/accelerating. It doesn't 
knock if it's parked and you revv it or if it's in 
neutral and you revv it. Only happens while driving. 
The friend that bought it has another motor to put 
in if he has to. But we want to see if we can fix this 
one first. What could the problem be? Thanks in 
advance.