Originally Posted by SPL

childers — There are two sources for the whine: (a) magnetostrictive noise coming from the motor-generators and caused by the heavy currents flowing in the coils, and (b) acoustic noise from the inverter switching transistors. The heavier the current, the greater the noise. The motor drive system uses pulse-width modulation, and the transistor switching frequency is changed as a function of the motors' speed, probably to prevent switching losses in the transistors from increasing as the motors' speed increases, and also to reduce the audibility of the noise generated. As I recall (this may be wrong), the switching frequency varies in the range of 1.25 - 10 kHz. The windshield is designed to absorb well in the frequency range where the human ear is most sensitive (~3 kHz). The whine is most audible with the windows open, and when the ICE's noise is lowest. If this has truly increased over time (I haven't found this to be the case), the likeliest cause in my opinion would be loose laminations or coils in the motor-generators. Toyota carefully controls the MG currents to keep things within design limits, and so it's unlikely that you have caused any damage to your TCH.

Stan