When in the high position the blower is powered directly from the harness through an orange/red wire with an in line fuse. Otherwise it is powered through a resistor network to cut down the fan speed. Since an increase in the fan speed brings on the noise, I'm thinking you have a loose fan, or there is something in the heater box that responds to the increased air flow, like a door not opening closing - that would explain your reduced air flow inside. Take the blower off and take a peek. Be prepared to replace all gaskets etc. Most supply houses sell a kit with all the replacement parts yu need.