Engine abnorma knocking noise

Hi,

I had a MK2 octavia with 1.6MPI engine, I decided to convert it into 2.0FSI, so i had bought my friend's crashed car

I had replaced the engine with everything under the hood ,the full electric loom of the car with almost all electrical equipments, all modules, front suspension, exhaust, and much more parts were replaced.

The damaged car was in a very good condition before it had the accident from the back side, i had drove it several times and it had no problems. (but the car had been stopped since 1 year).

The new engine had started on my car, but after two days I had the loud Knocking noise coming from the engine as detailed:

- The knocking sound doesn't look like knocking caused by poor fuel combustion.

- The knocking comes very loudly at the morning.

- The knocking happens when RPM goes between 3000 and 4000 then totally disappears above 4500

- The knocking happens at mentioned above RPM's only when full (or more than half full) throttling the pedal

- The sound comes from inside the engine.

- if i stop the car for almost 1 minute at idle, and then drive again, the knocking will not appear only for the first acceleration.

- If i drive the car toughly for almost 15 minutes the sound will never appear again till the next engine stop an restart.

- I had changed the oil two times with no advantage.

- No fault codes were recorded at the DTC memory.

- Two specialists gave two opinions:

1- There is a damaged piston.

2- Inside the intake manifold, there is something like gates with solenoid motors (something like the throttle electrical gate) which are causing this knocking.

Any one had this problem before?

Any Advise?

Thanks in advance.