Strange knocking noise from boiler

DaveB55

About 5 years ago we had the boiler at our building replaced. This building has 35 units and 4 commercial spaces, the replacement boiler is a Weil-McLain Model #1088 series 1B W/A power flame burner model # cr2-go-20b dual-fired which we run on gas.



There is a strange very loud noise coming from inside sounds like towards the bottom of the boiler about 1-2 minutes after the initial start-up. Also i noticed it is even louder when it is colder outside. After about 1-2 minutes of these loud intermittent knocking sounds they stop and the boiler runs ok.



I asked a boiler installer about this and he said due to the fact that we have a tankless coil on the boiler (this supplies heat to the one basement apartment) We would need to install a "blanket" on the boiler, cost around $2500. The technician who i spoke with at Weil-McLain also indicates that this may be required. I have no idea what we should do. If anyone has had this experience and can offer us some sound advice, it would be appreciated. I have enclosed some pictures. Thank you.