Several thoughts on this:
Based on your description of the blower noises (also posted in other threads), I'm suspicious that your blower motor might be on its way out. But "noisy" is a relative thing, so just a hunch. There have been previous threads about rebuilding or replacing the motor because of this.
I don't know for sure if the '74 is the same, but the blower motor speeds are controlled by a old school "resistor" (looks like a toaster) to reduce the motor's speed by increasing the electrical resistance. I've found that resistor is typically corroded and well worn so the resistance values are triple what they were originally. That means the blower is spinning about a third the speed it used to. When that happens it's time to replace the resistor. We've been discussing the possibility of using a modern style blower resistor pack instead of the old school "fire starter".
Related to that, the (high) amperage that feeds the motor is routed through a lot of circuits/components that don't handle the load well. That's where one of the relay mods helps.
As Karl stated the heater box tends to become very contaminated over the years. I've opened them to find all sorts of disgusting things. Not only was it surprising they worked at all, but a serious health hazard as well. Same for the cowl area - to REALLY clean that area out you may have to remove the wiper assembly, heater resistor, brake reservoirs, etc. In addition the heater box tends to develop other issues, like the control cable mounts break (so the doors don't move very far), or the door pivots break, or the insulation around the edges of the flappers and seams has deteriorated, and more. Need to remove it, clean it, rebuild it.
Even when everything is working correctly these aren't great ventilation systems compared to what you might be accustomed to with more modern cars.