Chrisjob has nailed it with the PWM suggestion. Almost any analog/VA/ or softsynth capable of PWM can pull that off. You may need to play around with high-pass filtering the sound as well, and some distortion would help.
Absynth has a patch calld DnB Junky that would be a good place to start as well. If you have it, and have a sampler, sample that patch and HP filter it, then distort it to taste.