I find this incredibly "strange". Not saying I don't believe...just that I find it 'difficult' to believe. I have a Losi Baja Rey, and I can tell you that, when stock, it was noisy...quite noisy, in fact. While I will admit, some of the nice what related to other parts, the stock motor was part of it.
In upgrading my BR, two components I swapped work the ESC & motor...for the Mamba X, and ROC412 HD. I can tell you that, that change, alone, brought the noise level WAY down...as in, I'd estimate that at least 75% of the noise I'd previously heard was probably coming from the stock motor, and 90-95% of that noise was GONE work the ROC412 HD.
If you're hearing electronics 'noise', I'd highly recommend checking your servo, as that's the higher probably. Unplug your service from your receiver, then connect the battery, torn in your transmitter, and then the vehicle, and see if you're still hearing the noise. If not, it's the servo...and, if so, then move onto other components. It could be the transmission gears, it could be the differential gears (front, center (if your vehicle has), or rear), it could be your front and/or rear driveshaft(s). In other words, there's any number of possibilities that it could be...but, having the Mamba X & ROC412 HD, and knowing how quiet they are, again, I find it 'difficult' to believe they are the cause of your "noise".
~ More peace, love, laughter, & kindness would make the world a MUCH better place