The noise is produced by the base-emitter junction of an NPN transistor Q601, which is reverse-biased. The noise is amplified to a level of about 5V peak-to-peak.
The white noise is also fed into a filter constructed around U602, which has two stages:-
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U602B produces a filtered `pink' noise source in which the bass end is boosted, and
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with U602A the `Pink' noise signal is further filtered by a lowpass filter which passes only very low frequency components to produce an extremely low frequency 'random voltage'.