Hi folks

my son has 2007 Impreza EJ204. 
radiator burst and engine over-heated.

radiator fixed but now have notice this engine knock which started a couple of weeks after over heating event.

Have attached video for help from all you big brained Scooby mech heads please:

 

cheers

Mark

Maybe cam belt tensioner

secondary air pump, black round thing? sounds too high pitched to be bottom end in the video

Thanks guys. I thought cam belt tensioner but seemed toward back, but will look. I did a fault reader code check and it threw up P2441 (secondary air valve failed closed)......so you might have something there about secondary air pump. Will look into that. 
 

appreciated

Ah, secondary air system is total garbage, takes water in and the valves in it eventualy rust and fail. Bad news is you cant just turn it off in the ecu, you need to get the valves removed so they dont jam in the wrong position.