Noise

[Milly]

What is the name of the crackling noise a
TV makes when there is no signal

[Annna]

Left over noise from the big bang????

[Clanad]

That's what it is and it's called Cosmic Microwave Background...

[SIROAM-ETAHI]

That is the noise of its case splitting as you smash the crap out of it in frustration when you discover that you have no signal.

[4candles]

AKA 'mush'.

[chakka35]

I suspect that very little of it is CMB. Mostly it comprises stray radiations from all manner of electrical sources, such as switches, electric motors, atmospheric static, spark ignition and so on.
With no main signal the receiver's Automatic Gain Control is inactive and the set picks up any old rubbish on that frequency. As soon as a proper signal arrives, AGC clocks in and the gain goes right down, suppressing the mush.

[mibn2cweus]

The audio for most TV channels is transmitted via modulation of selected frequencies in the VHF and UHF frequency bands. Tuners designed to receive these frequencies are inherently noisy in and of themselves when operating at full gain which as chakka explained is the case when no signal is present corresponding to the channel to which the receiver is currently tuned.

This noise commonly referred to as Johnson�Nyquist noise or thermal noise is the result of random motions of free electrons that are present in electrical conductors in proportion to the conductors temperature with higher temperatures corresponding to greater electron activity.

The input and early stages of a radio frequency tuner circuit receive the greatest amplification and it is there that most of the noise produced within a tuner/amplifier circuit is generated.

Some additional electromagnetic noise picked up by the antenna and introduced to the tuner may also contribute to the overall noise output of the audio system however even in the absence of external contributions typical TV radio-frequency tuner amplifier circuits generate most of the noise typically heard when no signal is present.