Vauxhall Zafira burning smell and electrical buzzing sound

I get a burning smell coming from underneath my car. When im driving i can hear a buzzing sound when I press the accelarator. when im stopped at lights the car will rev on its own only a tiny bit but it gives a buzzing sound. when i finally stop and get out the burning is very strong. It doesnt do it all the time which I find strange and when its not doing it the car drives fine.