My Marshall & Wendell Ampico upright had a squeak when I first got
it.  After lots of listening with my head in the bottom of the piano
I finally located the squeak.  It was from the wire rod that runs
from the "diving board" to the spring pneumatic where the rod passes
through a guide on the bottom side of the expression unit.  The rod
was very slightly bowed and touched the thin metal guide.

To make sure it was the source of the noise I applied a slight
coat of Vaseline using a Q-tip cotton swap.  This stopped the squeak.
I later rotated the rod to a place where it seemed to no longer touch
the guide.  While the rod was still squeaking I watched carefully and
never saw it stick in operation.  I had expected to see and hear some
disturbance in the dynamics, but I never heard any difference.

Bill Chapman in La Quinta, California -- The temperature was 120
degrees [49 C.] for a few minutes today until the storm moved in.