blaster knocking

There are many things that can produce a "knock"
loose cylinder bore to piston clearance
worn crank bearings
detonation
ect......
You need to try and figure out what area of the engine the knock is coming from.
 
could be detonation or loose bore then, hard for me to say not hearing it myself. You can use a long screwdriver as a makeshift stethescope to pinpoint the location of the knock. As a note, blaster engines are not the quietest motors even in perfect condition. How many hours are on the top end?