Could almost be Bart Simpson but seems a little too dark for Bart.
Everybody knows who says this, but you are all in a conspiracy not to tell me. Well, BWAAAAAHAAAHAAA, to all of you! (Actually, I find it rather irritating, which I think is the point. But until I hear/see the character who made it famous, I will never truly get it. It would be like trying to explain "Doah!" to somebody who had never seen Homer Simpson in an exasperated moment.)
Could it be just bwaaahaa? That would be Nelson Muntz from the Simpsons tag, every time something bad happens to somebody when Nelson is around (usually it’s caused by him) he points and expels a big “bwaahaa”.
Here’s his wiki
http://en.wikipedia.org/wiki/Nelson_Muntz
Any time I see someone do that, I hear Homer Simpson in my mind. I remember episodes where he would just bust out in laughter, and it sounded just like that.