Had a look at an FCXA this morning with the same fault. Its not the fan as I cleaned and checked this.
It appears to be a small solenoid or switch 'chattering' on the PCB behind the electric termination panel on the RHS of the heater, next to the fan.
I have tried cleaning out the solenoid contacts and the pcb in general, but its not made any difference. I think it would need a replacement PCB, sadly, and I dont think I'll find one anywhere.
The Maintenance Man, February 2021