I have just purchased a 44 year old home which has baseboard heat and I am totally in the dark concerning the maintenance of this system. I have noticed a knocking sound coming from one of the baseboards in my kitchen and also a smell I cannot quite describe when I enter my home, which I suspect is coming from the baseboards.
There is very little maintenance to be done to a baseboard heating system as long as the system remains sealed from leaking and the pressure remians high enough to keep the system filled, which in most homes 12 psig suffices. Once a system leaks it is no longer maintenance, but repair.
The water inside the system has become deoxygenated over time, so there is no more oxygen to corrode the iron parts. Keep the treated water inside and do not introduce oxygen.
If there is a banging in the kitchen baseboard, air bubbles are letting water slam into elbows so the air must be removed. If the noise is not a bang, but a ticking sound, it is the sound of the baseboard sliding as it expands in its hangers as the wter heats the pipe, this can be corrected.
The smell may be from something on the baseboard or may be an improperly firing heater. Something on the baseboard can be vacuumed and cleaned with spray cleaner.
Have your system checked by a well-recommended heating plumber to make sure you do not have dangerous combustion. At the same time, he can flush the system of air and do any normal maintenance.