Tachometer or Tach Cable

I've got a problem that seems to be cold-weather related, and I'm trying
to determine whether it is more likely the tachometer or the tach cable.
Happened a couple of times last spring, and once last week when we had
a cold snap in Michigan.

On the ground, I hear what sounds like an intermittant 'chirp... chirp'
sound. Sounds like a shaft vibrating in a loose bearing, kind of like
you sometimes hear when you fast-rewind a cheap VCR tape.

In the air, I get a drift toward an overspeed reading (2900 - 3000 RPM
instead of maybe 2700). Based on sound, power settings and airspeed, I
am convinced that the engine did NOT overspeed. After a few minutes
(5?) the tach settled down to "normal" readings and stayed there the
rest of the two leg flight.

Some people suggest it is the tach cable, tightening up and untwisting
due to either a kink or stiff lubrication. The tach, they say, is
reading the instantaneous velocity of the highest rotational speed.

Others say it is the tachometer (bearing noise) because they say "they
had that once and when they replaced the tach, the problem went away".

Tach cable is relatively cheap to replace, but difficult to thread
through the firewall and behind the panel. New tach is easy to replace
but more costly, and will add confusion to future record keeping in the
logbooks.

I sure would welcome counsel from people on this list who may have
encountered this problem.

Rich