Could be a lot of things. Things to check. Timing at 10 deg BTDC with E1 & T1 shorted.
Valve lash clearance on the intake and exhaust cams. Look in the cylinders by eye or bore scope. If there is a lot of carbon, try a octane booster to see if the knock goes away. If it does use a good cleaner in the gas like Lubro Moly "Jectron" found at NAPA auto. Could be piston skirt slap from too much clearance between the piston and bore.
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