What caused the VSI and ALT bouce in the IMC?

Yesterday I flew a cessna 172 in the hard IFR. When I penetrated
apparently a heavy cumulonimbus rain cloud area, the VSI and altimeter
started to oscillate and bounce +/- 250 FPM (ALT oscilated 200~300 ft
up and back). The rate of bounces was about 3 to 4 Hz (3 to 4 times
per second). It lasted for about 10 minutes until I passed that area.
The AI and airspeed was relatively stable in such light to moderate
chops condition.

Can anyone explain the possible cause of the oscillation? And did
anyone experience this before? If it was the heavy rain that caused
this, I still don't understand why. (please explain how can rain cause
this?)

The lesson learned here was to turn on the alternate air intake if the
airplane was equipped with one. Or from the book, "break the VSI
glass" in such condition. Well, I did not do it this time (other
than turn on the pitot heat, OAT was 35F). Next time if it happened,
I will break the glass (or turn on the alternate in a equipped
airplane).

Any thoughts are welcome.

-cpu