Mine has whined from Day 1 whenever MG2 is either driving the car or performing regen braking. The sound is easily drowned out by ICE noise, wind noise, road noise, vent fans, stereo, flatulent passengers, etc. so it's not surprising that you don't notice it all the time, but it is always there. The pitch and level changes depending on the electrical load. Easiest way to demo is with windows down on a windless day coasting to a stop. Change the pressure on the brake pedal with varying amounts of light force (vary the amount of regen braking) and you will hear the whine change.
The noise can also be heard on acceleration at low speed, but it's a bit trickier to notice. Again windows down works best, if you pull off a full EV start from stop then it's quite noticeable. Also, if you do an easy start from stop right after system start with a cold ICE, the car will often prefer to let the ICE idle and accelerate mostly from battery. The idling engine will be quiet enough to allow you to hear the whine. Anytime the ICE is producing power, it will easily drown out the whine and normal engine noise is all you will hear. That's why it's mostly noticeable during braking, because the ICE isn't producing power.
This noise is one of my favorite features of this car, I will often shut off the radio and vent fans just to hear it.

Two climate control systems, one inside and the other at the tailpipe.

2007 Camry Hybrid (in service June 2006)
FE: 6.67L/100km or 35.3 mpg (6.11L/100km or 38.5 mpg in summer)