I believe that's actually the attack on the chorus and/or reverb effect that was applied to the synth. It would sound the same each time that first note was pressed, but you can't hear it on the other parts due to interference with the other tracks and sound effects playing at the same time. This is why I love instrumental versions. You pick up on tiny details never really heard before.
No1 mentions how this is the 1st true G-Funk beat, before Dre left Ruthless he was already on that sound...Cold187UM was never on dre level not then or now, only DJ Quik was on Dre level