[wiredgeorge]

Why not remove the heater vent to see if there is any indication of damage; perhaps wetness or signs of wetness. If a repair is needed, it would be easier if you knew a little more to zoom in on better answers. Sounds like you have the same construction as noted previously in this thread based on age. I would also take a close look for signs of water penetration up top; Look in nearby cabinets that might have a line with water in them to start the investigation. Water can also get in via window seals and the like so check these things before ripping into the floor.