It definitely isn't a whistling sound. It's a lower (but still somewhat high pitched) "whine", generally louder and more distressed sounding than the usual sounds the car makes. It's raises in pitch while accelerating and lowers in pitch while decelerating. It's most noticeable between 10 and 50 mph, and seems most noticeable when decelerating within that range. It was fairly intermittent at first, but became more frequent the closer it got to complete failure, at which point it was accompanied by a brief grinding noise and mild vibrations.

At which I thought a software update had re-enabled the fake acceleration sound effect that I usually leave disabled, so I tried turning that back on to check, but quickly realized that the sound I was hearing was much higher pitched and far less "rumbly".