What causes the background hissing in car speakers?
Here is an article that explains where the noise comes from you are hearing. This is the detail below but some formulas have not been extractable from the source so read the source.


  Resistor Noise can be Deafening, and Hard to Reduce
  
  Q. My low-noise amplifier is not low noise enough. What am I doing
  wrong?
  
  A.  It may not be the amplifier causing the trouble; analyze where the
  noise is actually coming from. A typical operational amplifier circuit
  contains six uncorrelated noise sources (the smaller ones can usually
  be disregarded1). The amplifier itself has three separate noise
  sources: a voltage noise source appears differentially across the
  inputs; and current noise sources appear in series with both inverting
  and non-inverting inputs. Remarkably often the problem is not the
  amplifier, though, but the thermal noise generated by one or more of
  the three resistors that set the amplifier gain and provide bias
  current compensation. Analog Devices has over sixty types of op amps
  whose voltage noise is less than that of a 1 kΩ resistor .
  
  This answer is rarely popular; it is far more satisfactory to blame an
  imperfect amplifier and replace it with a better one than to admit
  that there is a fundamental problem with apparently simple components
  such as resistors. In fact, a remarkably common response to a
  diagnosis of resistor noise is to seek a source of "good" resistors,
  with "good" being defined as without thermal noise.
  
  This is impossible. The basic physics of resistance shows2 that the
  random thermal movement of charge carriers in a conductor always
  produces electrical noise of value  where k is Boltzmann's Constant
  (1.38065 x 10-23 J/K), T is the absolute temperature, B is the
  bandwidth and R the resistance. (We often express this noise in terms
  of spectral density, making the voltage noise .
  
  Such noise is known as thermal noise, or Johnson noise, after John B.
  Johnson (Bell Telephone Laboratories – 1928), who was the first person
  to observe it. It might more properly be named Johnson/Nyquist noise
  as it was Harry Nyquist (also at Bell Laboratories) who explained the
  physical basis of Johnson's observation.
  
  So anywhere at all that there is resistance in a circuit, whether it
  is carrying current or not, there is a noise generator with an output
  voltage noise spectral density of . We can reduce the noise by
  reducing the resistance (this may increase current and/or power
  consumption), but reducing the temperature is not usually practicable
  (if we cool a resistor from room temperature (298K) to liquid nitrogen
  temperature (77K), its noise voltage is still more than half its room
  temperature value). And, of course, we can't change Boltzmann's
  Constant because Professor Boltzmann is dead3.
  
  1 Any noise source less than 1/3 the magnitude of the largest can be
  ignored for most practical purposes.
  
  2
  http://www.physics.utoronto.ca/~phy225h/experiments/thermal-noise/Thermal-Noise.pdf
  
  3 Died 5 September 1906 at Diuno near Trieste, buried Zentralfriedhof,
  Vienna.