High Pitch Squeak

Driving to work this morning I heard a very high pitched squeak coming from somewhere outside and didn't think anything of it. Then when out in my car earlier I heard the same again, when I put my window down it was coming from the driver side front wheel and when I dabbed on my brakes it stopped.

As soon as i took my foot off the brake it started again. It only seems to happen if I'm going over 50. Car is under warranty until December but just wondered if anyone had any ideas what it may be before the mechanics try and fob me off?

Thanks guys/ girls!
This could be a hard spot in the brake pad material (which will wear out or can be cleaned off with wire brush etc) or you may have something like a small stone lodged in the caliper/pad area which moves around when applying the brakes.