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.
==============================
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

"Highland Ham" bravely wrote to "All" (11 Jan 06 09:47:40)
--- on the heady topic of " 1N4007 varactors"

HH From: Highland Ham
HH Xref: core-easynews rec.radio.amateur.homebrew:90890

HH Zener diodes are often used as wide band 'noise generators'for use in
HH an impedance bridge used in conjunction with a receiver.

I understand to generate the noise that the zener diodes are operated
within their normal breakdown current range. However, as a varactor it
isn't desirable to have a breakdown current flow. The only tiny
current that flows is a saturation current which is related to
setting up the P-N junction boundaries within the semiconductor.

M*i*k*e