[BrianG]

Interference can be caused by many things. Certainly high current wires, especially the motor wires which carry AC, can inductively "transmit" whatever PWM frequency the ESC outputs. The higher the current, the more interference possible. AM radios are most susceptible to this since the actual receiver commands are modulated on top of the carrier (AM) frequency, even though the radio frequency is usually substantially higher than the noise frequency (there are things like harmonics that come into play). FM is a little better for this due to the modulation scheme, but not perfect. The higher the frequency of the radio, the easier it will be to reject this noise since the noise is so far away from the Tx/Rx band. However, that same high current can be enough to drown even 2.4GHz out, or at least reduce the receiver's sensitivity. Antennas near or on carbon fiber or metal chassis can have degraded performance too from signal deflection and "echoes". Other radios in the area (microwave, cell phone, CB, TX, FM radio, etc) can effect range too.