Avensis whistling / whining

[Willss]

Have a new (2000miles 17 plate) petrol manual avensis tourer.  Really like the car in pretty much every way apart from one niggle.  It makes a really annoying high pitched whistle/whine/vibration (I know that's not really helpful at all!) when accelerating under load (not when accelerating hard away from lights etc just when accelerating in a standard making progress way).  

It does it in all gears but if I take foot off the accelerator or dip the clutch it stops so is drive related not wind or tyres.  The 'normal' acceleration noise of a petrol engine is there when the revs build but this is a high pitched annoying noise (which I'm convinced shouldn't be there)!  Doesn't seem to do it all the time and seems to get worse once the car is warm. Also worse when car is loaded, going up hill or AC is in on (but still does it when AC is off).  Obviously car is under warranty and I'm planning to take it in again (tried before but was silent that day!) but wondered if anyone else has experienced this and found a solution.