1999 Ford Escort engine knocking at 2500 RPM?

If I rev my Ford escort up to 2500 RPMs there is a knocking sound near the engine.

I had to replace the engine head and piston when a valve dropped and ever since then there is the rattle at 2500 RPMs.

It has a 2.0 Litre, 4 Cylinder engine and runs fine except for the knocking sound.

Any help?

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