F56 Cooper S whistling/screeching sound

After returning to mini's with my wife's countryman last year I got the bug and have just recently bought a 2014 Cooper S (still has warranty until November). I enjoy driving it although when pushing on the car occasionally makes a whistling/screeching noise whilst in first and second gear (seems to come from front left area). I have had it at the garage on 3/4 occasions and they can't find a fault. Originally they couldn't recreate the noise and thought it might be the clutch but if there was nothing wrong with it then I would have to pay for it (est £2k). We agreed to monitor it - The car then made the noise going to work so I stopped at the garage and the mechanic drove the car and believed it was the brakes. The brakes were a bit corroded so they were cleaned at my cost despite telling them it only happens in 1st and 2nd gear. They had the car last Friday and no evidence of the noise so nothing done.

I can change to 3rd and the noise stops and it returns as soon as I engage 2nd. I can leave the clutch in after moving to 2nd and it makes the noise. It also gets louder when the revs are higher. If I stop the car the noise seems to stop. After a while of more sedate driving and I engage 2nd again the noise isn't there any more????

Any thoughts would be appreciated - before I lose faith in the mini.

Thanks

Niall

Are there any driving conditions that you can always hear the noise or is it intermittent?

It is intermittent. LAst night on drive home I was pushing a bit on a back road with windows down and sunroof open and could hear the noise in 3rd although quieter. It seems to get higher pitched and louder the higher the rev range. However, there is no noise when the car is stationary and revving. When stopped I turned the steering full lock both ways and no change in the noise. I then turned the engine off and started again no noise. After a while of pushing the car the noise came back almost when the turbo discharged in 1st gear - drove for a bit at normal speeds and noise was there. Stopped the car switched off engine and it was gone?