Does anyone know what is that brief racket before the percussion comes in at the beginning? Is it water rushing and rocks falling or something?
It sounds like spring reverb from a guitar amp. If you have the spring reverb turned up full and then shake the amp, this is exactly the sort of noise you get.
Or an external Fender Reverb unit, basically a bigger version of the spring reverb tank inside certain Fender amps housed in a cabinet with more controls. If you either kick, bump, or jostle a Fender amp with reverb springs, that sound will come out. The Reverb Unit can be seen in any number of Beach Boys concert films or photos from the early to mid 60's, right near the backline of amps.
Danelectro did an awesome thing with a reverb simulator pedal they had on the market about 10 years ago: It had a touch pad that you could tap with your foot and the sound of the springs bouncing around would happen.
For "Diamond Head" specifically, whoever it is on the songwriting credits not named Wilson, Vescovo, or Ritz was apparently the guy who was messing with the reverb springs on the session to get those effects. Nice way to score a credit.
