star trek - What produces the sound of transportation?

Whenever people transport in Star Trek, we hear a distinctive sound which seems to be unique for each Trek series.

In-universe, where does this sound come from, what produces it and why is transportation followed by this sound?

Answer

WARNING: This answer contains speculative conclusions

Here is a list of canonically known components of the transporter:

Annular confinement beam

Biofilter
Gravitational compensator
Heisenberg compensator
Molecular imaging scanner
Particle lock
Pattern buffer/multiplex pattern buffer 
Phase discriminator
Phase transition coil
Primary energizing coil
Site-to-site transport interlock

Targeting scanner
Transporter console 

List copied from memory alpha

First of all we can deduce things, that do not make that sound: Since this sound appears not only in transporter rooms but also on target sites where there is no transporter equipment, we can safely assume that it is not produced by the machinery inside the transporter room. So we're looking for things outside the transporter room, things, that can make a noise some 40.000 km away.

This shortens the list to two items:

• 
  

  Annular confinement beam This is the "forcefield" that confines the matterstream. It would need to be present at the beam out site as well as the target site; transporter room or no transporter room.

  
  



• 
  

  Molecular imaging scanner/Target scanner The object needs to be scanned. It doesn't matter, if you beam it down to a planet or up. It must be scanned otherwise it cant be reassembled.

  
  

Now I would like to argue, that it cant be the scanner because

1. Scanners don't make a noise. A tricorder beeps, but the scanning itself is silent. If a ship is scanned this usually cant be heard either: It is detected by the sensors.


2. If someone tries to get a lock on a person, that person doesn't hear a scanner fumbling around in the vicinity.


3. In VOY "Prophecy" Kim states, that the targeting scanners work on the same principles as Klingon. Now as Klingon teleporters don't make that sound, it can't be the scanners.

On the other hand I'd like to argue, that it is indeed the annular confinement beam, because:

1. If the transporter chief looses the acb we often hear a change of the noise produced. Instead of a steady, clear ringing, we hear a "flickering" up and down noise, which sometimes can be helped, when the acb is re-enforced.


2. Scanners are only needed to find out the configuration of particles of an object. It might be needed, to reassemble it. But absolutely necessary are they only for the initial scan. If that's true, we shouldn't hear a sound if someone arrives on a planet. Yet that is, what's happening. If that would be the target scanner making that sound, we wouldn't hear it when that person departures the ship.

So coming to my strictly speculative conclusion: It's the confinement beam.