UKPassats

Hello all, im after a little advice please. ive purchased a 55 Plate b6 estate had it about 2 weeks. When i started it up yesterday i heard a high pitched alarm, not loud more annoying, coming from the rear of the car. When i opened up the tail gate it seems to becoming from the passenger side wheel arch location. The best i can describe the noise is like the ringing you get in your ears lol. I was still in the boot trying to locate the noise when it stopped but for the life of me i couldnt pin point its exact location but its deffo coming from the boot.
Would anyone have any idea what it could be ?
Many thanks.