I think you made a mistake not opening the heater box while out of the car. Integral to how it work, there is foam insulation which seals the flaps and makes it work effectively. They ALL need renewal.
Ohh but I did, have derust the thing and spray paint it, all the flaps have new foam. I did disassemble it totally and inspect the heater radiator for leaks. pressuring it. and did the break booster also who was rusted.