This is based on some hobby experience while being somewhat educated in the matter (I'm far from an engineer here),
There are basically two sources of noise for electric drive units. One would be mechanical noises (gearing, etc.), the other being electrically induced but partly mechanical in nature.
How applicable this is to the Model 3, I don't know, but an example with brushed DC motors: these are normally driven by "Pulse Width Modulation", or PWM. Instead of varying the voltage to control the speed, you very rapidly switch the motor on and off. Controlling how long it is on vs. off sort of approximates varying the voltage, thus where the idea of "modulating" the "pulse width" comes from.
When I say this is switched very rapidly, I mean well above what you could do with your hand and a switch. Between hundreds of time per second and many thousand - that is, many 100Hz or even well above 10kHz. Sometimes, the switching frequency is actually within the same frequency range of hearing as the human ear. If these hard on/off pulses do anything like moving electrical coils around slightly (they do!), they can actually create sound with these vibrations. This is more or less the same reason you hear the 60Hz hum in some places, since North American electrical is a 60Hz AC waveform. If you've ever heard a high pitch whine from a power adapter or something, they're varying voltage using a similar switching idea (look up "switched-mode power supplies"), and happen to be using a frequency in the audible range. Generally, higher power applications can shove around coils and such easier (what with the higher electromagnetic fields and such), which is why you generally hear the "hum" around bigger equipment. EVs are also very high power motors.
So. The Model 3, on neither side, has a brushed DC motor like in my example. They're more like 3 phase AC motors, but these can still have electrical noise. Not just the primary frequency driving the motors, but all the electronics included in managing that power and any secondary switching frequencies. The ~370V pack (about 400V maximum) is DC voltage, but needs to be made into approximately AC waveforms to drive these motors. Which is why they have "drive inverters" to do this job (another term in this context is "variable frequency drive" or VFD, a more industrial term). They have all sorts of switching electronics inside to convert that DC voltage to the appropriate frequency waveform (actually, 3 of them) at any given time to get the requested torque out of the motors at a given RPM. Any number of things in this process could generate audible noise, including the primary drive high-voltage waveform supplied to the motors.
That doesn't pinpoint exactly which bit is making the noises we hear, but gives a general idea of the causes of such noises. I find the "beeping" from the front motor most interesting because the motor seems to be coasting in that state. However, I wonder if it's something to do with the "powered neutral" that Tesla seems to use (to my understanding, they need to actively match things to emulate neutral - it's not using power really, but there would still be switching going on and such). I will make a strong statement that should be true 99.999% of the time: When you hear the front motor "beeping", it is not being used for propulsion. It is coasting. The rear motor is providing all the forward power.
Thanks! This is super informative! Do you have any opinion then as to why I could barely hear ANY motor noises in another Model 3 Performance, but it's very loud in mine? Faulty motor in mine or lacking proper insulation?
