I have a split-level house (6 steps difference) with hot water heat. Copper/ fins not cast iron rads.

Problems: there is an average of 5-8 degrees difference between levels (measured in the central hallway- some rooms upstairs are much warmer than others)

and over the last couple of days the pipes have started making this annoying squealing/whistling/gurgling sound- loud enough to wake me up at night (which is the real problem)

Is this due to air in the system or something else??