I wish I did know the model. A manual would be helpful. As this is a high ceiling, I would need a latter to reach it that I don't have. It's a standard "builders grade" fan that came with the house, less than 5 years old. The fan has your standard pull switch.
i.e. fast/med/slow/off
So power is supplied to the XPFM through a standard wall switch?
Any chance you have a "shaky" neutral connection to the XPFM (picking up noise on the neutral)?
Short of that,
OldTimer's theory of back EMF is the only other effect I can think of. Without knowing how your fan speed is controlled (capacitor, tapped motor windings) it's hard to say what the phasing/amplitude of the back EMF is doing at low speed.
OldTimer - nice fan diagrams. I'm having problems figuring out the first diagram though. Seems like the speed control table doesn't match the diagram.