hot water radiators

Air

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.

Probably the pump.