Blocking off an EGR valve won't cause an engine knock. Two unrelated issues. Maybe pull the plug wires one at a time, see if the knock quits on a certain cylinder. Depending on what type of knock it is, could be anything from detonation to a loose wrist pin or a broken piston. EGR is Exhaust Gas Recriculation and is designed to pump some of the exhaust gasses back through the engine to finish burning the gasses not burned the first time through the combustion process. IMO non of the smogger heads belong on a performance type engine. Lots of good heads on the market that don't have the emissions garbage built into them......