Checked system. The boiler is operating properly with the burner turning on and off according to the zones. There are (4) zones and a storage tank that is feed from the coil. When the burner turned on, there was a rumble. Checked the oil flow and transformer. Both were ok. Removed the nozzle assembly and the was a dark spot on the tip of the nozzle indicating that it was not spraying correctly. Replaced the nozzle and system is operating properly. Cycled the burner several times.