Knocking is one of the major issues on modern high compression spark ignition engines, and the actual challenge is how to effectively control its occurrence protecting the engine, without performance impact. Modern cars incorporate knock-sensor systems for engines to minimize knocking, which can maximize engine lifetime, increase power, and improve fuel economy.The choice of the sensor type may depend of a wide number of factors and boundary conditions as the engine size, market segment or fuel. It's very common to use for mass production engines a knock sensor with a wide range bandwidth in detriment of the tuned one.Wide range bandwidth knock sensors are sensible to all engine vibration frequencies, therefore, this knock sensor configuration implies in a more complex signal processing than the necessary for a tuned sensor. The occurrence of different interferences, mechanical and electrical may be observed in the point of view of the ECU and it may interfere in knock detection.This study involves the investigation of combustion pressure effects, valve events and fuel air ratio in the knock sensor background noise levels and their consequences in knock detection in two different natural aspirated spark ignition engines, a 2.4 liter equipped with variable valve actuation -operating with gasoline and ethanol in three different charges. The data are processed using Fast Fourier Transform (FFT) on Matlab and the results are plotted in function of frequency and time domain.