What causes the loud noise heard from an FM receiver in the absence of a signal?

In an FM receiver, the Intermediate Frequency (IF) amplifiers preceding the discriminator (demodulator) are designed with extremely high gain. This ensures the signal is driven into "limiting" or clipping, which effectively strips away amplitude variations and static when a valid signal is present. However, when no signal is present, these high-gain stages amplify inherent internal circuit noise and thermal noise to their maximum levels. The discriminator translates this random noise into the loud, rushing "hiss" or "white noise" you hear at the speaker. Because VHF bands actually have very little atmospheric noise compared to HF bands, the noise is almost entirely generated internally. This is why FM receivers use a squelch circuit to mute the audio output until a sufficiently strong signal arrives to quiet the receiver.