Sounds a lot like "kettling", which is when the micro-boil on the heat exchanger plates goes macro, with larger bubbles collapsing as the flow moves them off the heat exchanger plates. Sometimes this is due to too low a flow rate- what is the temperature difference between the incoming water and output water? If the delta is more than a 40F this is could be a pumping issue, perhaps a failing pump impeller.
It can also be caused by lime deposits on the heat exchanger plates. Liming is more typical on steam boilers, which always add fresh water as it loses some water as steam at the vents and blow-down of sludge. But it can happen in a 40 year old pumped hot water boiler too. Fresh water that replaces the lost steam or "old" water from an annual flush has minerals that inevitably get deposited on the heat exchangers. There are commercial products for deliming/descaling boilers that are worth a shot.
If the system pressure is under 12 psi at the beginning of a burn cycle it could also cause kettling, which should abate as the pressure increases. If it's 20psi at the end of a burn there isn't a whole lot of room for simply raising the system temp- ideally it would peak no higher than 25 psi, since residential boiler pressure relief valves usually open up at 30psi (and it's not a precision instrument.)
After 40 years of service it's already beyond it's expected lifecycle, so if the delta-Ts are in the 20-30F range, and bumping the pressure up 3psi doesn't fix it even after deliming it's time to figure out what should replace it. Simply replacing it with something similar in size is usually NOT the best approach, especially on a system with 4 zones, since the individual zones can't emit the full boiler output, causing the boiler to cycle on/off a lot, or even short-cycle (less than 5 minutes per burn, more than 5 burns per hour for cast iron), which puts wear & tear on the boiler and cuts into it's as-used efficiency. With some wintertime fuel bills in hand, run a
fuel-use heat load calculation at the
99% outside design temperature for your location.
If the replacement is fixed-output cast iron, it's DOE output should be no more than 1.4x your calculated heat load. Even 1.2x is usually more than adequate for covering cold snaps.
If the replacement is going to be modulating condensing boiler, the radiation on individual zones would ideally be able to emit the DOE output at MINIMUM fire at condensing temperatures.
The napkin math on that lives here.
