This is the 3208th most frequent Polish word.
silnika
"Silnika" means "of the engine" or "of the motor" (possessive form).
In this sentence, 'silnika' is in the genitive case, indicating possession or necessity (of an engine).
Potrzebujemy silnika do tej maszyny.
We need an engine for this machine.
Here, 'silnika' again is in the genitive case, associated with the noun 'dźwięku' (sound) that it describes.
Nie słyszę dźwięku silnika.
I cannot hear the sound of the engine.
'Silnika' is in the genitive case, used here to indicate the object of the action of repairing ('naprawą').