After reviewing the information given and having transferred to the site in question through an out-of-body experience, I have determined that the system is a small commercial steam boiler system with an indirect heating tank using heated boiler water from below the water line. Nick is most correct in this instance. However, the boiler controls are set to bring the water through the water heating tank exchanger only after the water has heated sufficiently to activate the reverse-acting circulator switch, so while the burner cycles, the circulator may run steadily. However, the circulator is making noise from wear due to sediment running through the inmpeller and cavitation from periodic approach to the boiling temperature. The boiler and piping need flushing and the boiler water temperature upon hot water demand needs to be lowered somewhat to prevent cavitation. However, this can not be avoided during the heating season, so more frequent circulator repair or replacement is necessary. The water level will naturally rise and fall as bubbles occupy some of the water space as the water approaches boiling, especially at the suction side of the pump where the pressure drops inducing boiling at that point as bubbles take more space than water.