All of the components in the signal chain generate noise. A large part of the noise is random noise, meaning that all frequencies are equally represented. A typical source is an opamp or transistor. Source for this noise is most often thermal. Resistors can generate noise. All active and passive components contribute. This type of noise is difficult to filter out. Balanced cables can improve the SNR by about 1.4x, by summing two signal paths for the same source signal. The signal we want increases by a factor of 2, but the noise, since it is random, increases by a factor of square root of two. Other types of noise have some frequency components such as "hum" which is at 60, 120, 180 Hz, etc. This can be filtered down, but never fully removed in the power supply. Noise is a very important topic, since it exists in everything we do, see, and hear. There are numerous guides on the web.