What kind of heating system do you have. Obviously it's Carrier and it's an Infinity system, but do you have a gas furnace, heat pump, dual fuel? The controller is just basically a switch with brains that can talk to other parts of the system.
Does it do this all of the time night and day or when coming out of a programmed setback?
The controller, depending on it's version and how it's been setup in advanced mode, can set the fan speed according to the time of day just like it can temperatures. You manually setting it to MED could be being overridden by this kind of setting.
You say the unit ALWAYS runs at high fan speed? Does it also indicate that it's in high fire (for a furnace) or high speed (for a heat pump)? Does it ever use the lower heat functions/capacities? Coming out of setback with a large temperature difference to make up the controller may be going right to high capacity mode to heat the home. This can be the case sometimes if the controller is set to not use intelligent recovery, but instead a time has been programmed in as when the unit should come out of setback.
As far as the motor itself making an annoying noise, you're moving 1957 CFM's worth of air at fairly high static in high fan mode. The motor may be contributing to the noise you're hearing, but I'd suspect that much air moving at that kind of static is drowning out any small amount of noise those motors make. That is unless you are standing right by the unit itself. 20 some years of working with the ECM motors that are in the Carrier equipment, I've yet to ever see one that made a noise you could hear over the air flow noises. But of course there is always the first time.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.