A clanking sound just before the boiler starts indicates the clank is not from the boiler, but within the piping. The clank comes from a slug of water following an air space or a mechanical object striking. If the sound occurs from the water moving, the circulator must already be running. So there are some possibilities:
1. The circulator is running all the time. The thermostat calls for heat in a zone. The zone valve opens permitting water to move against fittings. The zone valve end switch fires the boiler.
2. There is a delay device for the boiler controls after the zone thermostat calling for heat starts the circulator.
Whatever the cause of the delay, one clang may be because of the zone valve mechanism. Several clangs would be from water striking turns. The most likely cause is from bubbles of air separating from the heated water while still permitting a bubble to move ahead of the water column. Air is never totally eliminated from a system, but bubbles out and is reabsorbed as temperature and pressure changes in a sealed system. If the problem comes from gathered air, placement of components to prevent air gathering in locations and eliminating air at the locations is a cure.