Well, is that with no rear brake pedal or a whole foot-full?
The front brake lever actuates one piston on the rear brake.
If no rear brake pedal is applied, then I'm supposing the front tire is braking harder than the rear, especially with the weight transfer to the front.
If threshold braking is being applied to the rear tire via both the brake pedal and the front brake lever, then yes, you might have heard the rear tire howling just before lock up.
To my knowledge, I personally have never heard the rear tire howl - I'm too busy listening for the front tire to talk to me.
I've locked the rear tire several times in practice....not a good habit...hence practicing until I got it right, then continued practice until I (hopefully) can't get it wrong under duress.
Keep at it!
Steve