Squealing noise

Hi, my skoda Fabia 1.6 tdi estate has started to make a weird noise when I pull away in low gear. It only happens when the engine is warm, usually after about 20 miles or so and initially sounds like the beep noise a van makes reversing which then runs into a constant squeal for a few minutes until I either apply the brakes or increase the speed and change up the gears. Any ideas? It doesn't happen every time and is sometimes worse than others, like today.