Where do the noises come from if there is no signal in a Radio Receiver? ?

Question

If there is no external noises.

Answer 1

A simple resistor ( or even a wire) with no voltage applied will generate noise based on it's temperature (see link to Johnson–Nyquist noise).  Amplifying devices will amplify this noise and add noise of their own.  So even when no external noise is present a sensitive radio receiver will detect these noise sources.  

Answer 2

do you mean like static???

Answer 3

Noise is basically electrons slamming around randomly. Some (RF noise) as actually received by the antenna; this is noise generated by the sun, atmosphere, static, etc. But most of the noise in a receiver is generated by the active amplifying devices. That's why really low-noise devices are really expensive.