I never tested a zener when used as varactor but I think these have a
much greater reverse saturation current (even far below breakdown
threshold) and it is this that might cause comparatively more noise
than a conventional diode with a tiny leakage current. Well, at least
that is what the junction noise equations would seem to indicate.
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Zener diodes are often used as wide band 'noise generators'for use in an
impedance bridge used in conjunction with a receiver.

Frank KN6WH / GM0CSZ