Problem found and resolved.
My "mechanic/handyman/lawn care guy" showed up this morning. I explained the symptoms, my suspicion of cause, and gave him a physical description. He grabbed some tools, slid under the machine, and dropped the bottom panel. The problem was immediately visible.
The Honda tech who installed the Inferno heater had done a good job of securing the heat ducts with heavy duty zip ties but the vertical portion of the duct running to the dash outlets had stretched enough to come in contact with the front axle driveshaft. A hole was worn in the duct but it had not wrapped around the driveshaft and caused serious destruction.
I don't value the dash outlets much, prefer using the seat frame outlets. Repairing the damaged duct with tape would not be effective and I don't want to go to the trouble of replacing that duct only to have it repeat the problem I have now so we eliminated the dash outlet duct. We cut out about 2' of duct to prevent it sagging and contacting the driveshaft again and zip tied the supply end tightly around the wiring harness to block air flow from that duct.
A quick test drive demonstrated the heater works fine using the floor outlets and the sound of the duct flapping against the metal pan as the driveshaft rotates is gone.
Not a 100% repair but a functional repair. Thanks to those who contributed information and advice.