I have a 135Btu/hr Peerless boiler used for home heating. Approx. 2 to 3 minutes after burner comes on a knocking noise starts in boiler, fairly loud, can hear throughout home. When burner shuts off, noise stops within 1 to 2 minutes. If I completely drain boiler and refill, the noise does not reappear for approx. 6 to 7 days. The noise sounds almost like a “boiling” knock.