I hear a loud boom sound when furnace cool down and my furnace is new... should I be concerned

No HVAC expert here, but to ease your fears a bit until someone shows up, my guess, since you say when it cools down, that a large piece of sheetmetal is contracting and the buckling of the metal is what you hear.

I have to take your word for it. If there is any chance this is not possible or does not make sense, loud boom sounds should be checked right away.

Call the installer back and ask him to fix his error. Ducts and plunums have folds in the sheet metal to prevent exactly what you are hearing. The guy who did your duct work messed up on at least one panel

It is possibly the ductwork expanding and contracting-oilcanning as it is called. Did you hear it before the new heater? If not, call them back.

A boom sound at the furnace is a concern. A pop or boom in the duct work usually is not a big worry. Dirty return air filters can make duct work pop. Restricted supply air flow can make your ducts pop.