Hi. Trying to remember...
To squelch the 'white' noise audio of an absent carrier, a high pass filter and then a rectifier to saturate a transistor should ground/cut the audio. The normal audio (voice range) should not pass trough the filter. Is that right ?
The white noise level to rectify should be above the Vf of the diode, right ?

Why is there white noise when no carrier and thus, no (10.7MHz) IF present ?

 

There will be 10.7Mhz band white noise there with no carrier. Noise is always there, except in space, where no one can hear you scream
(It is in space too, actually)

 

There is background radiation from objects, uncorrelated to the tuned carrier frequency RF energy from transmitters/devices and internal circuit noise (Thermal, Shot, Flicker, Quantum). Some of the noise is likely to be in the tuned bandwidth of the receiver that the FM detector will see as random deviations from the center frequency and produce a random noise signal.

 

A highpass filter will not block speech but pass noise in an FM radio. An FM radio already has a highpass filter (de-emphasis).
What frequency will you make the cutoff of the highpass filter? Speech goes up to about 14kHz and the audio (and noise) is rolled off so that the 19kHz stereo pilot is not heard.

EDIT: I forgot to say that an RF detector is used in an FM radio to squelch noise and to blend stereo in to less channels separation which reduces noise.

 

Thanks.
Can you show a sample circuit (or link to one) that detects presence of the RF ?
Or did you mean demodulator for the word 'detector' ?

 

Most FM radios use an RF signal strength detector to drive the automatic gain control, blend stereo channels and do squelch..