I've had 25+ years experience with the cast iron hot water heating
radiators in my building, and have never heard them squeel.
I expect what you MAY be hearing is the iron heating pipe rubbing on
some wood as it expands with the influx of hot water.
Normally, in a building with multiple heating zones, the source of any
noise OTHER than a tap - tap - tap as the heating pipe expands and rubs
against the wood is the zone valve allowing heat into that radiator
train. The zone valve can be anywhere in the radiator train, but it's
normally installed at the upstream end.
Use a mechanic's stethoscope (that will cost about $15 at any place that
sells automotive tools to mechanics). You put the stethoscope on your
head and touch the metal probe to anything you think might be making
that squeeling noise. The closer the probe is to the source of the
noise, the louder that noise will be heard in the stethoscope. When you
touch the metal probe to the source of the squeeling noise, you'll heat
that same squeeling loudest in the ear pieces of the mechanical
stethoscope. This allows you to pin point the source of a noise.
If you have any zones in that heating system, my guess would be that the
source of the squeeling is a noisy zone valve motor.