Changed spark plugs
SKODA Fabia II Hatchback (542)

The knock in the engine is inextricably linked with the operation of the crankshaft, which provides engine speed. Accordingly, the faster the crankshaft rotates, the more often a knock is heard in the motor. Depending on the mode of operation of the internal combustion engine, the sound may be louder or quieter. It is important to accurately establish the relationship between the increase in the number of revolutions of the internal combustion engine and the intensity of the sound. 

During the diagnostic process, it is necessary to check at what point in the operation the engine knocks louder. It often happens that at a high temperature in the system (at the moment when the engine oil is the most liquid and increased in volume), the power plant knocks hard. In some cases, a knock is heard precisely when the engine is cold, and after warming up, the noise completely disappears or becomes almost imperceptible.

Hope it helps!
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Alan Baker from Noeldeasy

If your spark plugs had the wrong length, they can be hit by the piston when it spins. I read that in another instruction. 

Check this pic for motor see through to see how that can happen.

Compare the old and new spark plug so they are the same length.

https://www.cars24.com/blog/wp-content/uploads/2018/08/how-a-car-engine-works-engine-parts.jpg

Another things that came to my mind:

If you accidently damaged the thread or the thread holder, then there might be a leak now, which might be the cause for the sound, coming out for each explosion, through the leaking threads. 

I remember from the manual of my touran, it mentioned to tighten the spark plug with fingers only at first to make sure it’s properly going into the thread holder, not the wrong angle. Then when sure it is ok, by finger screw it in to bottom. Lastly tighten it with the torque measuring  tool to the allowed max X NM according to the maker. For my other vw car it’s 8NM per spark plug maximum to not damage  the threads and cause a leak.

A compression test would probably be useful to rule out any leaks. That tool is quite cheap, like $20 or £20 something.