So..... the duct-work vibration issue (see my earlier posts from midyear 2018 thru end of 2019 regarding my son's home) which was resolved by slowing down the furnace fan during Cooling Mode has returned in Heating Mode.
The RA drop to the furnace is 25x8. Assuming 1200 CFM (he now has the factory manuals for the furnace not me) in heating mode, this equates to 700 FPM in the return air drop. My old Trane Ductilator identifies 500 FPM as the maximum velocity recommended in suction inlets.
My guess of fan cavitation due to under-sized inlet duct, as well as oil-canning of the duct, seems very likely as the source of the duct vibration.